Depending on the situations, extra characters can just be dead weight who'll get themselves killed. That's how I've noticed it, anyway.
For the prevalence of single units, I'd say it's more of a matter of how powerful a single unit can get. Pair Up, unlimited leveling, high stat caps, certain skills, etc. all allow you to disproportionally dump your power into a small number of units, where it would be a waste in games where the most you could get is whatever you'd get out of being 20/20.
Regardless, Legault isn't nearly as good at this game as he thinks he is and needs to stop parading himself around as some defining standard.