I, too, have the max of 6 identical weights on my L6060 but no liquid ballast.
Helpful but not nearly as effective as a ballast box or other counterweight for FEL work.
The loader on my MX is mounted to the tractor about 2ft behind the front axle, and the rear axle is about 4ft behind the loader mounting point and 6ft behind the front axle which is what the tractor would rotate about if the back end lifted up. I'd be interested to see what the weight distribution is on the front and rear axles with ballast in the tires and 660lb of wheel weights and just the loader and bucket up front.
Anyway, I'm not disagreeing with you. Ballast boxes are a better alternative for some folks over a large and heavy implement out back, but like many things tractor related, they're a compromise. They're an additional cost over an implement that has another function other than just weight, and they can't be moved to the front of the tractor when weight is needed there. For some a better option is suitcase weights that can be moved to the front or the rear depending on the need. I wouldn't object if I found a decent used ballast box locally, or if someone gave me one, but with Rim Guard, wheel weights, suitcase weights and two 900lb+ implements I can hang out back, not to mention the turnover plows, I don't see a need for one at the moment. It'll be interesting to see what the OP ends up with.
When I first bought the M6060 it didn't have Rim Guard in the rear tires or wheel weights. I raised a bucket full of dirt to dump on the dirt pile and the rear wheels came off the ground a little. No such problem lifting up heavy logs or dirt after 1,440lb of Rim Guard and 660lb of wheel weights were added.