How is that useless? It's a tasty playstyle, seriously gives in to the fantasy of the hyper mobile damage dealer, and i can imagine wouldn't do too terribly with the complimentary talents to go with it. Metric fucktons o' Fury for all the Chaos Strike spam your heart desires.
Although, taking Fel Mastery with Felblade in a single target situation would seem kind of redundant unless Felblade wasn't up in those situations where you're ridiculously unlucky with procs... well, even then the CD of Felblade is reduced by haste, so taking Fel Mastery with Felblade would be kinda redundant after all i suppose.