I guess I haven't had a problem with this, because - aside from Wizards who discovered the same spells, and subsequently built similar derivative spells (or similar counter spells) - my Wizards have all developed unique spells.
That's what I'm trying to fight. Because, clearly, muggles cannot contribute to anything, as evidenced IRL.
Combat should be more interesting than, "roll combat, DC 15". And, in most systems, it is. And, IMO, this is a good thing. For this interesting minigame, IMO, everyone should get to participate, albeit not necessarily equally. And, happily, that is usually the case.
Thing is, replace "combat" with bloody anything, and you've still got my opinion (kinda).
That is, you want to convince the king to deal with the orcs. It should be more than "roll diplomacy, DC 25". There is - or could and IMO should be - a whole host of pieces to manipulate, from reputation to evidence to blackmail to his daughter to going through intermediaries. Show me someone who cannot participate in any of those minigames - or in discussions choosing between them - and I'll show you someone who cannot think. Even my sentient potted plant could have ideas, suggest not to feed the vegetarian steak, or laugh at Taserface.
Now, I say "kinda" because, many groups I've played with, some or even many of the players will only be a detriment to any minigame where they try to "help". So, optimally, they would do nothing, and not earn a "participation" ribbon… and, realistically, if they *did* participate, they would get voted off the island (ie, in character, there would be no reason for the other PCs to want to keep them around if they exposed both the extent of their detrimental stupidity and the lack of wisdom to keep their mouths shut).
But, yes, normally, both in a game and IRL, you want everyone to participate, to help, to pull, if not "their weight", at least as much weight as they can. And, yes, I agree, how much weight each individual character can pull should vary, perhaps drastically.