I'm seeing the MP range from the low 20's MP to 50 MP in some of the new cameras.
Aside from shooting speed/file sizes/whatnot...what are the advantages of having fewer MP on that FF sensor?
As best I can recall, all recent cases where a company has introduced a new sensor of lower pixel count than an earlier sensor of the same size from same company have been with cameras built for speed: high frame rates and low noise at high ISO speed settings. Most evidence points to frame rate as the dominant reason.
It
might also be that a well-designed bigger pixel sensor can have better DR and noise than any file you get by downsampling from a higher pixel count sensor to equalize resolution, but with Bayer CFA sensors, that comparison is tricky. It is not quite a fair comparison to down-sample to a TIFF or JPEG of the same pixel count as the lower resolution sensor, because that down-sampled file from the higher pixel count sensor has information for all three color channels at each "super-pixel", whereas the lower resolution raw file has only information for one color at each pixel; R, G, or B. So that down-sampled file probably still has higher resolution, ay least of color information.