It can hurt coop early into your career. It's much better once you're nearing the highest tiers in NG+ though. And by "much better" I mean compared to SL matchmaking from previous games. --- I am snazzier, hot, hot rant. Warily slight as. Croak rush, OK? Weirder, almighty make out. ::)
I find it fun sometimes to go for as low soul memory as possible and head straight for say, Iron keep to pick up titanite chunks and McDuff. Then provided I didn't kill too much I can get summoned for the Pursuer.
Then I hit him with my +10 weapon while co-op for 300+ while the host is hitting him for 50.