And, since it's a show, not only probable effectiveness, but also aesthetics and (in some cases) how "interesting" the design is are counted in.
For example http://www.robotwars.tv/competitors/.../sweeney-todd/ <-- way too light, thin armor, weak weapons, and basically a block of steel.
But it could move sideways, making it somewhat innovative/interesting.
Or Nuts. I don't think anybody expected them to survive the first heat, but they had googly eyes and bright colours, costumes, and gave a show and some amusement.

So yes, some of the criteria are not about what bots are the strongest.
But i guess the pure competition has to get sacrificed a bit for making it interesting to watch.
After all, that's quiet a bit of money in the arena (and for the arena itself) and everything, someone has to pay for it. So they need commercials. So they need people to watch the show. And not only Techfreaks, but they have to make it interesting for someone just zapping through.

But after that, when the Show has started, it is pure competition, the strongest survive.
So good luck, if you've got something innovative it's quiet possible they'd take you in for season 3 (i think rumors say march 2017).
But yeah, being able to use them is a big factor considering spinner heavys.