It matters, but most people make the decision based either on the leader or deep set party loyalties. It matters inso far as its a reflection of the leader. Occasionally a MP can boost the Party in a whole region but that's rare.
My point is if Nathan goes and becomes the leader of the BC NDP it won't cripple the NDP to be without him. We have tons of bench strength, Romeo Saganash, Francine Bovine, Alexander Boltrice, Megan Leslie, Libby Davies, Charlie Angus, Murray Rankin, Peggy Nash, Peter Stouffer, Mulcair himself, and many others.