There is absolutely no downsides to a well designed FFP reticle.
The upsides are-
1) Consistent subtention regardless of power. This does not just apply to LR shooting. The reticle is a ruler and there are many things that can be done with it besides windage and elevation holds (which are the primary uses).
2) Increased field of view. This goes along with #1. If I am shooting and spotting for myself the power gets turned way down. Anywhere from 6-12x depending upon range, recoil of rifle and position. Even during sniper matches, with one of the best spotters in the business as a partner, I very rarely shoot on more then 16x. When we finished one of the toughest matches in the country this year, my partner had never taken his scope off 13x for three days of shooting from 90-680 yards. I had stayed between 12x and 16x for every target but two or three. When hunting whether the shot is at 50 yards or 500 I want to get as much information as possible about the shot. Better then 80% of the time I will see the impact of the round even with an unbraked 300WM if the position is good. Most shots I take are between 6x and 10x no matter the power of scope or range.
I've never met anyone that's learned and used a good FFP scope want to ever go back to a SFP.
