I don't know enough about Merrill's game to make a fair judgment on the acquisition.
But Habs are clearly looking to solidify that 3rd pair. They've had Romanov there and rotating between Mete, Ouellet, and last night, Leskinen played there. Merrill has almost 400 NHL games under his belt. That's more games than Mete, Ouellet, and Leskinen combined.
Habs have more pressing needs, particularly a mobile Top-4 LD, particularly one who can play with Weber. But that shouldn't mean you can't look at steadying other areas of the roster.