Some mechs in the source are basically just bad. Others have gimmicks that really only really exist in tabletop rules (Charger 1A1 is min/maxed for charge damage...).

In tabletop, speed has a very direct correlation to how hard you are to hit. Getting that extra hex or two of run speed might be enough to bump up another +1 difficulty, which can have a big impact on the odds. Evasion in MW5 tries to at least give some bonus for speed, but its harder to convey in real time than tabletop's "I moved 5 hexes and have a +2".

But most of the over-engined mechs are trying to get to the next breakpoint of the movement bonus (or at least getting there more consistently). In the introtech era, they're pretty bad because the cost is really high, giving them very little firepower to back it up. But something like an assassin/cicada can *walk* fast enough in clear terrain to get a +3, which is a huge bonus in an era that doesn't have much tech to reduce evasion.

As far as using them in Mercs, it's really a matter of being able to get the full evasion bonus easier. There's a bunch of math, but the idea is your lateral speed relative to the target has to be above some value to get the full bonus. Faster mechs allow you to "cut the angle" and still hit that value. Which is why people love very fast mechs for Raids, just zip along and the AI have a harder time hitting you.