i agree that I wish peter still contributed to the riff writing. Since opeth has always been a guitar-riff-centric band (versus bands that prominently use the bass or have drummers of standout personality), i think they lost something when only one person started writing the melodies. It seemed particularly bad on the(se) last album(s), when in an interview peter basically said all he did was solos this time (obviously, he must have done some more, but thats still a telling sign).
basically, no matter how much a single guitarist can try, he will almost always retread ground, be it rythmically (opeth: guilty) or melodically (also guilty here, but not so much). thats not even considering the structuring/transitioning of songs, which can get too samey with the same writer. Its a very, very rare guitarist who wont do this... and even they do, but then its called "their style" because of their prestige as famous players
so yes, i wish they wrote more together. these are nagging points... i still like opeth and all, and they have more variety than many bands. its still good now, but how long can it last...
p.s.-- spare me your "its your opinion" truisms.. yes... DUH DUH DUH.. its obviously my opinion!