Yeah, it looks impressive to have a bunch of warriors up there but serves very little to the game. And after having a couple stampede rolls go against me and see several of those warriors fall to their doom I have to say it's not even worth "protecting" them by having them up there.
I agree that, if you have the points, giving the Mumuk repelling lines and putting some tough warriors up there would be great. Use the beast to get where you want, drop everyone off and get away (so you don't end up squishing your own men if you stampede
). Such a force of a few Watchers, Half Trolls and Spearmen for support would be a most unwelcome surprise in the enemy lines after having just been damaged heavily by a charging Mumuk. That would probably take a game of 700 points or more to make really worth it and I haven't played Harad that high yet.
Normally I'd say only put your bowmen up there. If the commander is killed one can take his place and in the mean time they can help address nearby cavalry or support troops. If you are short on bows then several basic warriors and invest in the "Rocks" upgrade. That helps a lot for dealing with situations when someone tries to charge you...again maybe focus on the spear support if any.