Oh gosh, you can be more effective using cross-class abilities in a class system built around using abilities from other classes. That's so horribly broken! People also "need" Second Wind/Invigorate/Ferocity/Raging Strike/Prime Conditioning/etc. By "need" of course I mean the same thing you mean with "needing" Sentinel. You can win without it but it's harder, so everyone will want you to get that stuff. If you don't like it, quit 'til 1.21.