Personally, I'd guess sorcerers were limited that way to prevent players from asking "If I don't need a rose petal/pinch of sand/live spider/bat dung, why would I need an expensive pearl/diamond dust/big honkin' gemstone", and since several spells are balanced in large part by their costs, I could see a sorcerer with, say, Stoneskin freely available as being quite overpowered. (On a similar note, I think Eschew Materials dropped the ball in a similar way, and as such would've been better if it raised the spell's level more for more expensive components. But I digress.)
If you think that your sorcerer, or your wizard even, would be better served by alternate components, I say more power to you. If you think your Sleep spells are better served by poppy seeds and chemical mixtures for your Identifys, do what feels best for your game's mood. But spells from any source would be complicated and prone to abuse if they just listed the component gold value, and as such, the current system is as good for generic flavor as any. I could see a blurb to this effect in place for D&D 3.2, but until then, it's a fair agreement for everyone.