I think it's quite possible that on many seats in many vehicles, using both systems would not present any safety disadvantage. As mentioned, it may not be tested and therefore is not recommended and, of course, it may not offer any benefit, either. It is not impossible that the use of both systems could indeed cause the forces to be applied to the shell of the seat in a way that would be different than when tested one way or the other. You would hope that would not lead to a failure, but you never know unless it has been tested...
This has been discussed among manufacturers and others in the industry, as many are looking for reasons to do this or not to do this. As is often the case with many issues of this type, we are stuck waiting until someone spends the money to do the testing before any manufacturer or agency recommends using both systems at the same time.
Carseats are designed to be rigidly installed in the vehicle. The less movement, the better, though that is subject to diminishing returns. Keep in mind that the harness of the child seat will also stretch to some extent, just as the seatbelt can stretch. Rear seatbelt systems generally do not have the load limiting features found in front seats, except in some luxury and newer models. It's difficult to say where the tradeoff is between a rock solid installation in terms of reduced excursion and the benefits allowed by some energy management feature, such as a the load limiting features in such seatbelt systems. Some new carseats do have load limiting features included and this will also probably be a trend in the future.
My understanding from some carseat manufacturers is that rigid LATCH systems are probably not significantly better than the higher end flexible LATCH systems (those with two separate straps and adjusters) in frontal crashes. On the other hand, there do appear to be benefits in side impacts, especially compared to seatbelt installations and flexible LATCH systems with a single strap/adjuster.
Ultimately, if you have a good installation with a seatbelt, regular LATCH or rigid LATCH, your child is going to be safe if they are restrained correctly. Rigid LATCH may have some benefits, but I doubt the reduction in overall risk is significantly more than just having your child properly restrained in the back seat. Of course, rigid LATCH may make it quite a bit easier to get an excellent install, and that is important, too.