Honestly I think it's important. If only because if you suck at transitions, and the opponent is good, then he can get side control on you (or a better position) and then you can't submit him. Remember, the transitions secondary helps with getting your transitions, but also in defending your opponent's.
Also I think if your fighter is really bad at transitions, then he can get grinded out by a grappler with better transitions. Such a grappler could simply get positional dominance on your fighter en route to a decision.