There's an important difference between combat logs and chat bubbles. While, as you say, combat logs are not necessary, these logs are a very important tool players can use to test various aspects of the game, and give feedback on the balance of combat. Figuring out the game's damage formula, mitigation formula, and other mechanical workings of the game (which require a combat log to figure out) also helps in the creation of class and skill guides.
In games which feature a robust combat log, we see substantive discussions pop up on the forums about class, skill, and overall combat balance. These discussions feature real number crunching and math proofs as opposed to wild guesses based on anecdotal evidence ("I once saw this HUGE floating number above my head, so that skill must be broken!!!")
It's like having a public test server. It's not necessary, but developers who opt out of offering these sorts of tools to their playerbase are being foolish, IMO, and missing out on a golden opportunity to have their fans help improve the game. There are plenty of gamers out there who are willing to help test and refine the game beyond the official launch date, if we are given the tools to do so. The combat log is one of, if not the most important tool to make available for this purpose.
-Travail.