Is there any negatives if you play co op? I never looked into it that much..
can you co op any time? or only boss fights?
do you not gain as many echos? or items? possible not gain a trophy for soloing it?
The cost for doing co-op is that you need to use insight to do it. If you have no insight, you can't do co-op. Also, if it's anything like the previous souls games, the boss' defense/health scales with the number of players, so if they die you'll be left with a substantially tougher boss.