I used to play all classes for months and switching to them back and forth especially for PvP and WvW to have a knowledge of all of them when I have to face them against. The so called multiclasser.
I played on my cousin account, but now I decided to buy my own and I just chose 3 professions out of the 9. Ranger which will be my PvE main because of the pet tame which is cool, to do all maps and open world, but sometimes PvP and WvW as well, then a Revenant to purely PvP and finally a Mesmer to PvP and WvW.
As you see I chose 1 heavy 1 med 1 light profession as my 3 mains and I don't bother playing the others because I simply don't enjoy them. Having 3 is also good for meta changes and not to feel bored of playing over and over the same profession and nothing else, which happened to me a lot.