I'm definitely of the opinion that unmerging should exist (hence the revive). Unmerging should require that you have the minimum starting value (25 for archers, 50 for human swordsman, etc) in BOTH units. Every play Lords of the Realm 2? That's how it works. You cannot create an army in LotR 2 without at least 50 men, so you cannot split an army unless you have at least 100 men.
Also, the ability to merge units and then fire off their new ability is so very very abuse able. I understand how it makes the code easier to write and for the server to manage, but I have used it to widdle down stacks of 4k+ strength with 2k strength units until they were dead. (I stacked elven archers 1 at a time into a 150 stack, firing each time. I had a lot of mana, and the valour from each kill allowed me to buy more).
I definitely wish we could come up with a solution that prevents that kind of abuse on the mechanic.