Purchased unit should be immortal, or atleast should be revived by coins (not by crystal) and have fast revival time (5-10 minutes only).
I'm not sure if they are already immortal but in case they are not, maybe they should be.
One of the reason why I didn't bother buying the elven archeress and the orcish berserker is that I'm afraid they might just die later, which would be a waste of money to invest on. I know they have better abilities but the regular units can manage on their own. Having a hard time to revive purchased units is a liability which greatly decreases their value and makes them only useful in defensive roles.
If purchasable units become easier to manage/revive, I will definitely consider buying them.