Whilst not being an ideal subject for Abathur to ult... Muradin does decent damage and definitly cannot be ignored when he is beating on your back line or peeling the front... so if your team needs the cc and peel, then ult Muradin.

A lot of Muradins traits are situational...

Level 1, 3, 16 help Aba's Ult Evo a lot.

Piercing Bolt might be better than skullcracker (level 7)...

Level 10 situational...

Level 13 is situational... the burn might be better.

Level 20 go upgrade your chosen ult or go rewind... it really depends on what your team needs (cc and lankiness (upgrade ult) or damage with lighter cc and tankiness (rewind).

Abathur Build (warning, wall of text and ramblings):
http://www.heroesfire.com/hots/guide/abathermite-wall-of-text-w-ramblings-clone-locust-strategy-5261