The major down side to having individual units get EXP is that it requires micro to keep that particular unit alive. From what we’ve seen so far, I’d say it’s too early to tell for certain whether individual units will get experience. However, I think in lieu of individual experience, they’ll be using a complex itemization scheme to enhance individual units.
One interesting variant would be to have class-wide experience. Your individual archers contribute to the experience pool for the archer class and eventually that group EXP is what unlocks the next craftable item. For example, after the archers kill some number of air units (or get some amount of exp), they eventually realize that a different style of bow would be very useful against aerial units so they talk to the carpenter and design a specialized bow, which can be given to an archer and you have a new class Marksman.