So the goals has scored have been super clutch goals when the team needed him most...
Hes streaky what else is new. Your a shark fan and should be familiar with how he scores in bunches. If Marleau could do that consistently he would widely considered one of the best players in the league. NBC would be demanding shark vs penguin games to have the Crosby Marleau match up. Marleau is not that good. He has never been that good. He has never had the potential to be that good. He doesn't have the hands, for example do you really think he hits the goalies dead center so often on purpose? He doesn't have the dangle abilities like you would see from Datsyuk.
The way to defend against Marleau is to keep in front of him and never let him skate with the puck at full stride. Also make sure he can't set up for the one timer. If you accomplish this there is a good chance you will shut him down.
A fair comparison to Marleau would be Hossa. Although I would argue that Marleau is less streaky and overall a more effective forward.
With that said yes Marleau can do better and I believe he will.
You didn't lurk for long before signing up did you? Otherwise you'd know I'm not known for being one of Marleau's biggest fans. I just thought it was interesting.