So I WAS thinking the same as Arnaud and Leungbok, turbos are not really necessary at the fast speeds in TT and speed lap, HOWEVER, here is why it is a necessity.
----you cannot reach some of the shortcut routes on some tracks without them. This means you have a hard to get to faster route, but without a turbo jump, you cannot reach it. This means you can only use in in a head to head weapon race, that is it. No need to elaborate more, they are needed----