While carrying AA guns increases your general ability to deal with aircraft, and you can be more independant as a result and not having to rely on BW's and noobs in the CV section. However if you leave those T slots empty that works too, and in some cases you can actually keep speed if you don't. Just be ready to dodge dive bombers and be spotted if there are no AA boats nearby... But choosing not to do so is not a bad decision, as many players will do it regularly, and you won't get called out as a noob in BB4-6 if you don't have any.