What was the design philosophy behind rovers?

They ended up as a bit of a default. This probably happened because other factories were designed by where they sit in relation to Rovers. Hover has more extreme units, and many of them are glass cannons. Tank is generally heavier and tankier.

Rover is actually about speed though. They are generally faster than Hover, except for the three fast Hovers, so can field a more rounded speedy army. And they are designed to be more efficient in terms of raw stats than Tank. So I would say Rover is about speed and efficiency, but has a particular weakness in scaling. If you let then get a midgame advantage then they can end up just out-trading you.

I did a stat sheet, so here are some things I learned (bonus):
Mason - 69
Scorcher - 111
Fencer - 84
Ripper - 63
Ravager - 89
Dominatrix - 59
Badger - 56
Impaler - 60
Crasher - 111

(Dart - 153)

In short,
Fencers get slowed down by rippers,
Scorchers get slowed down by fencers.
Rippers get slowed down by dominatrixes, dominatrixes get slowed down by badgers.
Crashers have the same speed as scorchers, and can hang out in the same group.
Ravagers get slowed down by rippers, while scorchers significantly slow down ravagers. Ravagers and fencers have about the same speed.

From this, I conclude: Ripper is ideal escort for masons, badgers, impalers, and dominatrixes, to make use of high speeds.
Scorcher is the best escort for fencers, ravagers, crashers, as ducttape riot.

Darts are bad: They clog up build queue, they die too fast, they are hard to control, ignore them. Scorchers are a far cheaper raider. (Exception: Assaulting lone lotus turrets & unprotected mexes, soaking instant-hits)

This should help me get the most out of their speeds.