It's hard to say anything on this. I would say its unconventional and an original idea, but then so was all The Doors music. It was a very interesting and original experiment, that perhaps didn't have quite the result that they wanted, but is still a great album. Anyone who doesn't like it because of the added instruments has a rather limited mind.
I love it when instruments are put into music that they are not normally used in. I'm still waiting to hear a sitar in rock
There was a reason that the songs are attributed to Robby or Jim. In Tell All The People, Jim didn't want to sing the line "Can't you see me growing, get your guns", so they had the idea to attribute the songs separately so that everyone would know Robby wrote that song. I guess they just did the same to the others to be consistent.
Anyhow, it's a great album, a very original idea on their part