Are altcoins useless?
If mining and exchanging in to bitcoins can be called useful then no, they are not useless.
Potential for pump and dump is endless, speculations are inherited, morally doubtful but beneficial.
So do we really need altcoins?
We do. Mostly because competition that they create for the main actors. Without them established coins could get this "hegemonic attitude". When market loses its competition aspects, its stops being free and as such can not represent adequate value.
Every theatre needs the stage, main role, action and background characters to project and influence changes in to preferably dynamic main role. It may be a little troubling, as people don't like changes very much but that's the true and there is no escape from this simple analogy.