I am surprised more people don't go strictly by the book for material components. Why: because they are a vital balancing aspect to the game. That is not to say that only components for expensive spells are important either. It seems to me that it is not always possible just to go back to town and restock (after all, this is not a roleplaying video game). Rather one ought to keep track of exactly what components one has on his/her person so as to know exactly how many times a certain spell can be cast.
IDHMBWM but let me point out a few examples:
Shield Other requires both the caster and the recipient to be wearing a platinum ring. If either gets hit with acid, the ring is probably toast, which would end the spell. Also, unless everyone in the party had the ring, the cleric can only cast the spell on certain people. Finally, an enterprising rogue with a bit of spell knowledge might try to take the ring off in combat with pickpockets while invisible.
Take any spell with a natural component, like say a blade of grass/hay, or some such thing. Consider a group that goes into the underdark and gets trapped there.
Consider any spell that has a focus (not something that is consumed by the casting), whether it be arcane or divine. Even if the focus is cheap, without it no spells.
The reason why I say that not just expensive components, but all components, are a necessary balancing factor is a product of how I game. When its "vs. spellcasters", there are certain key ways to combat them: silence, grappling, prolonged damage, etc.
One of those ways is to strike an object and go for either the spell component pouch (hopefully separating it from the spellcaster) or the specific component in hand. When doing this unarmed, the object would then be in your hands.
Finally, if you are not keeping track of components that you are probably not keeping track of the move equivalent actions necearry to retrieve the components for casting spells. When components are stored anywhere other than in your hand, in a Heward's Handy Haversack, or in a Glove of storing, it would require a move equivalent action to get the components from your pouch, spell component pouch, or wherever, per the retrieving an item rules (hence the rule about not being able to cast any spells with M in a grapple unless they are in hand). Again, this is a very necessary part of balancing spellcasters as they are by far the most powerful characters in the game. Again, there are ways to get around this (and I would rule, as with a weapon, that you could retrieve spell components in a spell component pouch while moving), but they are still there.