If we're talking about his actions in DOA, well, ummm, Hayate hasn't done anything too impressive (yet). Hayabusa won the 2nd tournament, Ayane "won" the 3rd, Kasumi was the one who defeated DOA5's story boss.
it's not that he didn't just win. It's the facts that
A) He was literally bitchslapped into a coma
b) He was way more badass as Ein than he ever was as Hayate
c) He's officially the weakest and most useless of the four ninjas.
d) HIs only role since recovering his memory was to send his men to their deaths en masse trying to kill Kasumi.
Ok, Hayate is a jobber, but, i have to correct those:
a) He was blasted against a tree and broke his spine.
b) Ein never did anything too impressive either.
c) He actually won against Kasumi twice if you go by itagaki canon, and DOAD also adds his collaboration towards Omega.
d) He actually is the one that led the ninja attack against doatec in DOA 4 and DOA 5, being the responsible for the disruption of part of doatec in some ways.
Honestly speaking we haven't really seen all that hayate has to offer in terms of skills so you can't solidly say that is the case. We need more of him period ! Yes he is a main character but besides being a clan leader we don't really know that much about him
I agree that we need more Hayate in the DOA series, and if other characters went playable on NG games, he could be there as well (nothing in the campaign, just an extra playable character), but Hayate needs to do more in DOA because his portrayal in the games is poor and he hasn't shown anything that a leader should do.