From the point of view of St Mirren, and other similar clubs on the 'footprint' of Rangers and Celtic, this HAS to get in the sea.
I'm not saying it happens all the time, but we have picked up occasional gems that have been discarded by the ugly sisters. McLean and Morgan are the obvious examples, though there are others. We've also had some successful loans from them, too.
If the colts teams were operating at league level, that source would dry up. What's more, it'd be harder to attract young players to our squad because they might see the colts route as a 'better' chance to progress, by remaining within the setup at those clubs.
I'm not saying any other club should care about where we get our players from, but this will hopefully be reason enough for Saints to vote against it, before you even consider the multitude of other reasons.