As far as the sense of making things into something the PC's fight, well, what else is a monster good for? I mean, how often do you ever see dryads or unicorns in adventures? It's very, very rare. I'd much rather the monsters in the books be applicable to the widest range possible. It's far easier to turn a combat monster into a non-combat monster than the other way around. IMHO.
Any monster should be able to fill three roles. One of them is Adversary. A monster entry should be interesting when it is fought, but it doesn't need to be likely to be fought by most adventurers, just a significant subset.
The other role is Ally. A monster should be interesting when it chooses to side with the party. Again, it doesn't need to be likely allied with most adventurers, just a significant subset.
The final role is Anybody. A monster should have an existence beyond the PC's, a reason for existing outside of encountering them and killing them.
Without filling all three roles, we're left with stinkers like the Phantom Fungus, which is an interesting Adversary, but is hard to fit (at best) into the other two roles. When we fill all three roles, we get things like the Drow, which has gone FAR beyond it's original inception as "anti-elf."
The reason we don't see many dryads or unicorns in adventures is more a feature of how adventures are designed than any inherent flaw in the concepts of the creatures. For instance, in the second installment of
The War of the Burning Sky, there exists a dryad AND a unicorn, because the adventure makes heavy use of the themes of the fey spirits. The Unicorn is part of the reward offered to an intrepid party. The Dryad is a key component of "saving the day" entirely. Fey isn't a very common theme in adventures, and neither is "temple of goodness and light."
But I think part of the reason Fey haven't been very common is because they neglect to be interesting, quick-running Adversaries, by and large. 4e has a real chance to fix this, while keeping their functions as Anybodies and Allies intact.
If the monsters are to be applicable to the widest range possible, they need to fit all three roles. A Good Unicorn obviously has a function as an Ally, and as an Anybody, it is, as Rich Baker pointed out, an avatar-spirit of goodness, purity, and light. So then the only thing left is to make it an interesting Adversary so that parties who DO fight it have a fun time of it. And given 4e's warlocks and tieflings and evil paladins and Asmodeus-worshiping PC's, I don't think it will be as rare of an occurance as it has been in previous editions.
So there should, IMO, never be "noncombat" monsters, or "combat" monsters. Monsters should be able to do both. And it is (or it should be) EXTRAORDINARILY simple to take a Good Unicorn, strip the alignment off of it, and make it a monster more fitted to be an Adversary for a wider range of groups. Heck, arguably, Eberron's "no absolute alignments" rule already HAS done this.
