Why does the ICS dialer have letters next to the numbers? It would make sense if it had T9 built in, but it doesn't. So why?

They should really implement that btw. I know it exists in custom roms.
I would assume it's because there are still phone numbers that are distributed with convenience letters (e.g. "phonewords") in them, and there are probably still automated call answering programs that use them. The letters on the numpad far predate T9. 

For the US specifically, Wikipedia also has an article that explains the history of telephone numbers and the various ways that letters have been used over time (it was different in other countries, but I can't find a detailed article for that at the moment).