A plastic stock on a muzzle loader is the gun equivalent to; "yeah, but she has a great personality". I'd keep it for using in the woods, and banging around, but wouldn't use it much otherwise.
Part of the appeal of ML'ers (for many) is the link to the historical past. A plastic stock loses much of that connection, but it IS practical.
For others, the main reason they're in them is merely to offer another option for their hunting seasons. In that case, the historical aspect is of lesser importance. So where I'm going with this is that YOU have to decide what's important to YOU, and make your choice accordingly.