Yes modular and hydra creatures auto-benefit, but don't forget spikes!
Both this and Spike Rogue work well with Protean Hydra:
Repeatably pay 
to get 2 more counters. (just 
after you play your heartstone)
Or curve a turn 3 suspended Fungal Behemoth into this to attack with a very large behemoth. Maybe your first or second turn drop was a graft creature too, so that it can receive the behemoth counter, and still graft to the behemoth so that this root-kin gives him a counter as well.
Don't ignore the fact that all that synergy is above and beyond the efficiency that this has naturally.
Compare with another strong card with similar applications: Fangren Firstborn
Posted By:
kiseki
(8/11/2011 5:20:12 PM)
great card... useful not only with other graft but in a modular deck, or hydras
Posted By:
tantallum99
(4/22/2011 9:13:14 AM)
This card also pairs nicely with a card plucked from the Plane of Lorwyn. Vigor really can turn Cytoplast from a game changer to a game ender very, very quickly.
Posted By:
Ralnakor
(2/20/2010 12:32:42 AM)
He works nicely with the new Simic evolvers; he evolves them twice and resets them for later use.
Posted By:
Stinga
(1/28/2013 5:52:07 PM)
Simic mutations at its best.
Posted By:
SavageBrain89
(5/30/2009 4:07:55 PM)
a 4/4 for four mana that pumps all your creatures, in the right deck
Posted By:
Arachibutyrophobia
(10/14/2011 7:25:15 PM)
Proto-proliferate, and likes proliferate.
Posted By:
CastleOrange
(12/14/2011 9:08:39 PM)
Argggggg too much text...
Posted By:
land_comment
(3/5/2011 11:07:57 AM)
Your dog might eat it. .5.
Posted By:
Anathame
(8/31/2011 7:38:20 PM)
if u can swing the mana cost this is a nice combo
Cytoplast Root-Kin + Mayael's Aria + mycoloth
+1/+1 to all your Saporlings, Cytoplast gets +1/+1 each Saporling, Mycoloth gets all +1/+1 from Cytoplast, and because of that makes more Saporlingsnext turn... Oh! and because of the Aria, when Mycoloth or Cytoplast get more then 20/20, u win.
Posted By:
Zyant
(8/18/2009 5:48:18 PM)