The fact that Adrian was "let go" as opposed to him leaving of his own accord is a moot point really, as it is a well known fact that he was not happy with the musical direction that the band was heading towards with No Prayer. I still have a Maiden fan club magazine for the time shortly after Adrian left, where Nicko says "his heart wasn't in it anymore" (in regards to the initial No Prayer sessions which H was a part of), which is the reason why there was a mutual parting of the ways. If Maiden hadn't let him go, it's highly likely he'd have left of his own accord anyway, as he was disillusioned with the band at that time wanting to go into a rawer, heavier direction, whereas he wanted to write and play more melodic material. When you compare "Silver And Gold" with "No Prayer For The Dying", it's very clear that H was not on the same page musically as the rest of Maiden so a split was inevitable I think, even if Maiden didn't decide to let him go.
As for the calibre of the material which Davey has written, I wouldn't say everything he's written has been great, but the vast majority of the songs he's contributed to have been great. For the most part, he has contributed "album tracks" (as opposed to singles) many of which, have been criminally underrated, such as "Judas Be My Guide", "Fates Warning", and "Still Life". And in more recent years, he has even started contributing major hit singles for the band like "Rainmaker", and "....Benjamin Breeg". Plus, "A Thin Line Between Love And Hate" is one of my fave Maiden tracks ever, and one of Davey's greatest contributions to the band's catalogue.