One of the members at my club uses the DEX210 in MM4 only - our club track is clay (
no blue groove)
And he's a front runner at both club level and scores well in the nationals!
He couldn't get the RM3 layout to work nearly as well as the MM4 layout even on lower grip tracks. One of the things he plays around with on his car are these rear chassis weights - that, and about 10g of extra weight between motor and transmission. Other things he works with are lead plates to shim in front and/or behind the battery and different sizes of spur gear to move the motor to the front or rear.
I don't have a DEX210, but I too found weight balance is a pretty powerful tool. From what I've been told/taught, it is not just the weight balance that is important, it is also how is distributed over the chassis. Placing weight in the extremities of the car increases inertia. Turning/changing direction will be slower, and a lot of weight far at the back makes the rear tend to slide/sweep out - this means oversteer can less easily be corrected when it occurs. So if you have a powerful motor in your car, don't mind putting more weight in the car but more towards the center (to end up with the same change in weight balance overall).
All in all I would say, if you can find a fellow 210 owner who can borrow you some weight sets (of whatever kind really), grab the chance anf try it out! If you can't, I think they're still nice for in your toolbox. Indoors you may not need it unless your track is tight and has low grip sections (like wood), outdoors it will prove useful in the wet, or on dirty astroturf (I was on a track last year that had some algea growing on parts of the track - it stayed moist and slippery over the whole day even in sunny conditions).