In principle, Antless, that is a perfectly workable idea. Lighter framework "pursuit" divisions, so to speak, are quite viable. Not sure what the downsides would be. Interesting idea. I would imagine they might be more susceptible to air attack, being in motion, away from the herd so they would have no defensive bonus. Don't see why that would be particularly dangerous for them, though.
Aha! One downside just occurred to me, though I think its relatively minor. It is rare that an entire frontline retreats in unison. So probably the first units you routed would have to be allowed to retreat, with you holding your pursuit forces waiting for a larger hole to be forced. Then when that hole enlarged and you sent them in, they would have to bypass some fights still in progress. So they would have still fighting defenders behind them, some defenders would arrive at their retreat targets before them, and it would look a bit chaotic. I dont think the early retreaters would be a problem, since they would not have had any time to recover. But if reinforcements SRing in from elsewhere also arrived at roughly this time, it could get really messy for a bit. Not necessarily dangerous, just confused.
Still, a nifty idea to play with. Good one.