Like you said in the original post, you have the choice to replace; are you fine kiting with 5 shaman as is? Keep 'em, you'll still be able to pvp just fine. Instances giving you trouble? Swap out for the pala for a real tank and get ready to write a fair number of tank-healing macros should the need arise, and it will.

Bottom line is it comes down to your choice; we dunno your lvl either, I'm assuming 70 so there's nothing to fall behind on really; if it's lower than that and you DO decide you need the tank, who cares about the leftover shaman? lol