Yes, his songwriting is pretty weak, but to just say that he is awful is to ignore the reason why so many people knew him. His voice was absolutely incredible, even at 67.
His voice was nice, but that does not make him a good MUSICIAN; he was an awful musician. He was clearly a terrible musician. With such a great voice, he still managed to create a lot more bad works than great (I do not want to get into whether Black Sabbath counts or not; Dio Black Sabbath was shit to me, but that is a pretty divisive argument). Compare that to someone like Freddie Mercury who was cranking out something like Barcelona (the album) right up to his death bed (not that Barcelona compares to anything from early Queen, but still).
Altmer, please. Rainbow has at least some odd 10-15 songs that are extremely enjoyable if you like hard rock stylings, and Holy Diver/Stand Up and Shout/Rainbow in the Dark/We Rock are all great. I forget which other song I liked on his first album, there was one more.
Also, if you want to compare Deep Purple and Rainbow, the sound was probably more mature and "meaningful" in Rainbow overall, guitar and otherwise. Deep Purple were always trying on too many styles, and while it produced a few amazing moments, it also produced an incredible amount of songs that just could not suit Gillian or Blackmore at all.