You're taking a strong anthropic principle hardline here, and I'd suggest more of a weak anthropic principle. Our societies are like they are because of the path we've taken to get here; it wasn't a necessity to get here. There are many ways that these societies could have and do diverge.
I'm thinking in more general terms; of how the ability to task specialize requires cooperation. Without task specialization, a society can only progress so far. Farmers, smiths, masons, craftsmen, and most roles in society are interdependent with each other. And you REALLY don't get philosophers, scientists, and the like without a vast number laborers in the same society. The time it takes to master deep calculations takes away from being self-sustaining.
Even assuming the species can "sun" there is still the task of obtaining trace minerals and water, as well as the martial tasks of policing and soldiery. Possibly even hunting. (Not necessarily for food, but for trade and protection- there ARE big, dangerous herbivores out there, after all.)
Wealth & Status, social role, etc comes from an earlier role in protection from exposure: if you have the wealth to afford clothes, you're better off than the person who doesn't have the materials goods to afford it. This is a natural extension of that. If our plant society doesn't need clothes for protection, then they wouldn't develop it as a status symbol. A different set of status symbols would develop, if status is important at all (I'd argue that it well could be, due to the strong individualism that many plants play; colonial plants – the inspiration for the Killoren/Wilden and many other plant-people – are actually rarer than we tend to portray them).
Highland New Guinean tribes use penis gourds as clothing. It isn't about protection, it is all about politeness, status and tribal identity. And in some cases, about carrying goods hands-free.
Your point about pockets is important though. Finding different ways to carry things may well be key here. But these are plant people. Couldn't they have extra vines that wind around items or grasp things in a way that's not wieldy like an arm, but is good for securing things to their bodies until they need to release them? I'm thinking almost Ficus-level vines. Of course, there are many different kinds of plants, and vines are not present in many taxa. But it might be an interesting substitution.
Certainly! Everything physical in their culture depends at least in part on the details of their anatomy.
Finally, the protection point holds merit as far as the plants may want protection from the environment. However, if we are doing tree people, then barkskin is the norm, so we don't have to worry about light wounds that clothes provide some protection against. And if it's a tree person, bark is also a great insulator. We may be talking like Warforged – clothes may not be necessary (though Warforged in Eberron are part of a society that that does have clothes as status symbols so some 'forged do wear clothes).
They could still suffer from exposure, though, especially those who travel. Too cold, too dry, too hot, too wet, etc.- all extremes have detrimental effects on flora as well as fauna.
Also woodpeckers, boring insects, plant blights, and so forth. Fire?
I take issue with your "Advanced beings" line of thought (you're suggesting that we're better than plants, which is a big anthropocentric assumption, especially when we're trying to talk about plant societies).
Really? Really?
I'm suggesting "advanced beings" in the sense of being sentient. All that other stuff about being mobile year round, having no forced hibernation periods, having language, mathematics, science, etc. is just gravy.
But your point about light energy is important. Plants don't need to feed these extremely energy-consumptive organs we call brains, and they don't need to spend energy on locomotion (at least not usually). Growth, synthesis, and reproduction are the main consumers of energy in a plant's organ systems. Reproduction is by far the most expensive. If plants had to locomote, then they would need a lot more energy. We might imagine that plant-based "humanoids" might only arise in sun-washed countries, and they might have massive solar-fuel cells (perhaps even other plants that are slaves to the plantoids) to save up energy for when they're running low.
Or they just need to drop their leaves or retreat underground for the colder seasons. Think like a plant, not like a human. You might not have plant adventurers travelling in colder climes. Then again, perhaps you'd have coniferous plantoids in those wintery places.
Hibernation/retreat just makes their communities more vulnerable the more mainstream sentient societies that are fully mobile year round and scrapping for resources.
Above ground AND in the Underdark.
Think of their environment, not just them.
If they are not at least as active as animal species during autumn & winter, they're at a competitive disadvantage.
Exactly, which is why I suggested them trading solar-food for useful nutrients produced by their Myconid neighbours. That said, they may not need wood for building: perhaps they grow their homes by nurturing other plants or have their ancestors die and hollow out into their homes. Fuel may not be necessary; some plants might hate fire. Though that's a bit of a cliche; many plants love fire. Those redwoods out West would die if the small forest fires stopped burning. And many blueberry bushes only come in after brushfires (there are too many examples to count). So fire might be a resource nurtured, but used responsibly by a plantoid community. I could even imagine your oils for dyes idea – but imagine the plantoids stealing the oils from other plants? I think it would be less about agriculture and more about harvesting, with a plantoid people.
Growing a structure arguably requires
greater skill and agricultural sophistication than our methods.
If
nothing else, your structures would be competing for the same resources as members of your society. That means some sophisticated irrigation and civic planning.
But without straw, wood or coal, you're very limited in what you can craft. Forget bricks, for one. And almost any kind of metalworking.
Many of these may or may not be needed. Interesting questions, to say the least, but I could think of dozens of counterexamples where our plantoids would get around these quite naturally.
Cisterns & silos let you plan for the future and tide you over during hard times.
Bridges give you access to resources, trade partners, and routes of travel/escape.
And I find it hard to imagine that plant people would prefer being exposed as opposed to having shelters against droughts or storms. Why have your leaves whither in the hot sun or be tossed like an oak by a mighty wind when the humanoids around you show you it doesn't have to be like that?