I don’t know about need, but it is a pretty great module. Teletype with expanders can do a very useful subset of what Marbles does, but perhaps not everything. I also think they complement each other nicely and it’s good to have Teletype free to do other things.
My first reaction to Marbles (I thought I was only going to be testing Plaits!) was “what the hell is this thing? That looks like scary math stuff on the front panel. Deja Vu, is that a French joke or a Matrix reference? Oh, random… I don’t need random.” But I use it on a regular basis now. I suspect of all the 2018 Mutable stuff, Marbles will sell the fewest units because it may be hard to appreciate until you’ve tried it – but that people who have one will love it and use it a lot.