I've done it but would definitely not recommend it across the board. When a case comes up where you need it, you'll know exactly what you're doing and why. That's important, because it can be dangerous and or deleterious to your rifle.
I used BPI plastic shot buffer to fill a case when using a reduced load. I didn't want to drive the (cast) bullet any faster, but without it the pressure was not getting high enough to give a consistent burn. I used Quickload to approximate the effect of buffer, by adding the buffer weight to the moving mass of the bullet, and subtracting it's solid volume (found from density of the material) from the case volume, and choosing a combination of powder and buffer weight which took the predicted pressure above 10k, but not by very much. I convinced myself that it was shrinking my groups a bit, but looking back I'm not entirely sure, plus it was a pain.
If you just cheerfully top up a case without doing the calculations, you risk overpressure. There is also an enigmatic phenomenon called chamber ringing, where the explosive pressure of the charge acting against the buffer is said to bulge the case in a very localized ring, over time distorting your chamber and making extraction difficult.
In effect, the requirement for buffer says that you are using an inappropriate powder for the calibre. There can be times where that is unavoidable, I'm limited in the range of powders I can obtain (can't get Accurate/Lovex, Vectan, for example) and with reduced loads for cast you are always off piste to some extent.
AYA SLE 12b, Harrier FAC air, Sako Finnfire .22