When I used to trade forex heavily it was useful to have 1 subaccount but only for very long trades (usually when very far stops and collecting overnight interest) just to calculate different strategies and not to mix with day trades and scalping etc.

Other than that I don't see any point in subs. I also don't see the point in many exchanges. Open a few for sure to see, choose one you like, done. Unless you treat trading like casinos and are using all of them to do promos and bonus etc.