1. Your bards have to use magic for diplomacy? Why? And how is magic more effective than non-magic for diplomacy. This is a game-world/GM problem. Charm is the worst way to make friends and influence people because it eventually wears off.
2. Invisibility and flight is better than non-magical sneaking (mostly; of course the DM should still be having the character make Fly checks with the result modifying the subsequent stealth check). Until the spell wears off. But rogues can also play this game and turn invisible. But after the spell wears off, they still have the edge.
3. The wizard can outdo the fighter in damage for one or two rounds, but not for much more than that. But seeing as how they used some of their best slots for outsneaking the rogue, that's not going to happen today.
4. Fair enough. But I have seen some people try to make magic be too good by making it always work as desired, which, by the RAW, it does not.
I play a Pathfinder Bard in our Skull&Shackles campaign. I've got enough Use Magic Device to be able to use just about any scroll I can buy - and our GM is very liberal with magic item purchases.
And still I find that I use the skill every time instead of a spell on a scroll... Why? Simple - all a matter of action economy. When the brown goo hits the whirly-thingy, it is much better to climb or jump
now, instead of flying
next round...
As for skill points, we've always house-ruled in 3.0 and 3.5 that INT increases are retroactive, so the Pathfinder rule is no stranger to us. However, when the wizard gets an INT item, you get an item that increases the attribute
that is appropriate to the skill and this raises the max in a way merely more skill points are unable to do.
Of course, the item creation bonus is shared with all in the group - same cost for all. When there are multiple casters in the group the creation feats are often distributed - "you do wondrous items, I'll do wands, and she'll do weapons...". (In our current campaign we instead have an equal number of additional NPCs each that benefit all with their creation feats - the advantage of having a ship with a crew...

)
As for sneaking, I've found in practice that there are just too many ways around it - monsters with scent, blindsense, tremorsense, etc, etc, that it simply makes a healthy stealth skill the safer option - even more so with increasing level. Also note that invisibility as far as I recall is not absolute in Pathfinder. It is just a major bonus to stealth - so if you don't have a decent DEX score and stealth skill as a base, well, it's just going to take you so far. And the stealthy character will probably get that +5 stealth item, since it is her speciality, while the "rule them all" wizard hardly can get a +5 item for
every skill... (Oh, the slots!

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