Having played against more than one gamer who felt that anything less than an Assault 'Mech wasn't worth fielding, and any more than a single mapsheet was a waste of table space, I realize that infantry are not for every player. If you want to play Turretech (where's the registered trademark symbol when you need it?), there's nothing more that you need to learn about the game or tactics, and all you need to do is roll big numbers. If you want variety, mental challenges, and something to teach you new ways of looking at and dealing with the situation, then the more options the better. Infantry significantly increases the options.
While Infantry can be semi-effective against 'Mechs in the right circumstances, they're potentially lethal to vehicles, even though outclassed. I'm not too thrilled with playing just 'Mechs and Infantry, but in a 'Mech, vehicle, and infantry (with APCs, or at least better than "foot" mobility) combined arms scenario, you've got a lot of stuff with potential to affect the situation at any given moment. Granted, it takes a lot more time and thought to play out, but it's generally worth the extra effort.