The numbers of branches staying open late are getting less, and probably, at least partially, for that reason.
I've always questioned the need, though. I'm sure there are some exceptions, but for the most part I think all they're doing is enabling procrastinators... and I say this as a procrastinator. Tax Day is like Christmas and your mother's birthday, it's the same day on the calendar every year. It doesn't sneak up on you.
I don't have a source for this, but I have been told that many years ago the deadline was March 15th, and many people filed last-minute, so they pushed it back to April 15th, and the same people didn't file until April 15th. Hence, my thought that it's just catering to procrastinators. I believe that if you allow midnight, they'll scoot in at 11:45pm, and that if you tell them normal closing time at 5pm, they'll scoot in at 4:45pm.