Those trained units are just cannon fodder...you wont be getting too attached to them anyways, once tactical battles are in and creatures are mowing them down by the dozens.
Hmmm so trained units won't get XP in the final version at all? This is weird. What if I train a unit which has a full adamantite armor set + a Godslayer +15 weapon. ..? Ok this sounds silly, but you got the picture. THAT unit shouldn't be a cannon fodder! 
Can we make changes in the XP system via modding? IE. Is it possible to modify it, so that the trained units will also gain XP and level up?