I think there are two separate issues here where weapons are concerned. You have weapons that are great for the SP game, and weapons that are great for the MP game - but often these do not coincide. To balance the weapons in MP, you would also affect SP, and possibly make SP less balanced, and vice versa.
I think what they need to think about is possibly limiting the types of weapons available for SP and MP, and use some kind of modifier to balance the weapons better for MP mode.
If you take Unreal Tournament as an example - they took some of the Unreal weapons, and balanced them for Multiplayer use! The Unreal weapons worked perfectly well in Single player - but were not suitable for multiplayer.
I am not saying that all weapons are affected in this manner, but some probably are. It would therefore make sense to me to look at SP and MP as two different games (which they essentially are), and balance one set of weapons for SP, and another set for MP. That would probably alleviate many of the problems.
I've not really played MP (bad net connection), but in SP I didn't really use the conc rifle or rail detonator that much. I think they limited the ammo for the rail detonator, which was okay by me. I only remember using it to destroy the AT-STs anyway.
And that raises another point. I think ammo should continue to be limited for the more powerful weapons, so that no-one can just blast through a level (SP or MP) by just using one weapon (except for maybe the lightsaber).
BTW, thanks Wilhuf.