All men are created equal, and so too, should your characters. That's my opinion anyway.
Sure, some of your characters don't have as much to say or as important a role as others, but (for you anyway) they're all people. They all deserve to be heard. You've spent the time working on your plot line and what's supposed to happen in your book (let's all face it, not everything goes according to plan), why would you skimp on your characters.
What happens when you give one character two days of your time and half a notebook for their thoughts and personality, and the next gets two minutes? First of all, WTH? How is that fair? If you're like me, and the characters are voices in your head the second you think their name, they complain and gripe, a lot. But it gets worse! That second character whom you've devoted little to no time to becomes flat and boring.
If you think that's bad--there's more. If you don't take the time to listen to each character, you end up with a cookie cutter version of the first. Every character you create from then on may look different and have a different voice, but aside from one or two traits, they are the same person. All of them become flat, bland, and completely unremarkable.
Now to step off my soapbox, I have the sudden urge to say, "And this has been a public service announcement." *sigh*