Cool idea! Of course, if you're going to track how many you have, you could also track things like experience and attrition...
An alternative method could be to instead make it so that each card in the Starter + Reinforcement set is all you get for the whole campaign. As in if your 4th Longbow unit is eliminated in the campaign you're out of Longbows.
I think this is a very interesting concept... now its no longer a simple chuck the trash unit out there and who cares if they die - you could run out of cheapies by the end! It would certainly make you more attached to specific units.
"Argh! That was my last Troll!"