More practical droids?

[Batman]

Or they already have a gazillion of the things so why not use them as long as they perform adequately?

[biostem]

Though on that specific issue, there likely is a problem with military grade small scale repulsorlifts. Why else would R2 and Jango Fett use thrusters and a jetpack respectively instead of repulsorlifts?

Repulsorlifts only work near a gravitational body, like a planet - if you wanted your astromech or personal jet pack to work in space, then you'd need a rocket-like system instead. We don't see battle droids being deployed in any significant number in space, (except for special-purpose ones, like the vulture or tri-wing droids), so this really wouldn't be much of a concern.

Or they already have a gazillion of the things so why not use them as long as they perform adequately?

That would make sense for TPM, but from AotC on, we see that they're running production lines of the various battledroids, so I don't think that still applies. Of course, I guess you could argue that it'd be quite involved to retool those production lines for new designs, (though I'd argue that something that amounts to a droid head with a blaster on a small repulsorlift should be simpler to construct).

[Batman]

Err-I was talking about the B1-crewed tanks?

[biostem]

Err-I was talking about the B1-crewed tanks?

Oh, my bad.