What makes M-Venasaur so great isn't just his offensive capabilities, but his defensive stats matched with his offensive power. Synthesis (a reliable form of recovery) helps accentuate M-Venasaur as a tanky pokemon. I'd say Synthesis is THE most important move for M-Venasaur to have. Coverage is nice and all, but it shouldn't supersede the necessity of healing for a pokemon such as M-Vensaur. He can definitely take a hit and Synthesis allows him to stick around for a while.
even when i did the standard physical/special defensive (and mixed) that this thread suggests, using appropriate personality and everything. synthesis did nothing but use up the other players PP. or i could take the hit and counter and kill it with better coverage.
like i said maybe im using it wrong, but everytime ive put synthesis on his moveset it either sits there with me staring at it or i use it and simply have venu die a turn later or maybe 3 with no way to offensively attack.