In Faster Than Light (FTL) the Rockman has extra health and walks slowly but is that all he's good for. A really slow tank doesn't seem that great to me.
Rockmen also are immune to fire, in addition to their health bonus. This makes them ideal for repairing systems where fire is spreading. They do still have to breathe, though, and will lose health if there is no oxygen in a room.
There are also a few events where you are guaranteed a better result if you have a Rock crewmember, due to their unique physiology and abilities. There's a full list of these events on the FTL Wikia.
Finally, don't discount the 50% more health - this gives a Rock crewmember a bonus against fighting other races. In situations where combat is at a higher priority than movement (say, a boarding party) this can make a real difference in a crewmember's survival. They won't outlast a Mantis, but can defeat any other enemy crewmember in one-on-one combat.
To expand on agent86's overall answer:
The fire immunity also makes rockmen ideal for assaulting other ships, especially when paired with a fire bomb.
The biggest challenge in boarding is the fact that those ships that have a medbay puts you at an immediate disadvantage. This is solved by fire bombing their medbay proceeded with teleporting in your rockmen boarders. This puts you at a very great advantage:
After a while, when the enemy's health is low, they'll run away from the fire and not attempt to repair any burning systems as they'll die from the damage.
As a bonus, it just so happens the second rock ship starts out with a fire bomb.