This is tricky. It's tempting to support legacy browsers, but if you do too good a job of supporting them, you don't incentivize your users to ever get their sh*t sorted, and upgrade their browsers. It's a vicious cycle I am eager to avoid.
Yeah, but when your "users" are more properly called "customers" -- or even more important, "potential customers" -- then some web dev's desire to preach the gospel must take a back seat to doing the job the way it needs to be done, rightly or wrongly.
It's fine to push for strict browser standards when the only people who will ever see your web applications are within your own organization. Public-facing sites are a different matter.