validation is nice as a personal standard, but in the grand scheme of things, it's a tiny issue. as long as it behaves correctly without alienating a lrge percentage of potential users, people are happy. the two big reasons being:

• most clients are technically inept and have no idea what you're talking about. the good ones just do whatever you tell them.
• lots of back-end code does stuff validation doesn't like (specifically .net)

there's probably a lot more, but i'm drawing a blank right now, it's late.

it's nice to do, if you can. if you're going to do it, you might as well do it right, or as best you can, but time = money (spent) so unless you're working on some kind of government/organization (charity/foundation) site, most won't deem it worth breaking your back over.