Not necessary on a sporting 870 as birds and bunnies don't shoot back.
It is necessary on a defense weapon such as duty, hd or bear defense. For all the talk about training resloving the issue, don't care how much you train when the SHTF and your A** is on line you can make mistakes. I have as much training as anyone here and more then most, mostly because I am 60 and it is a wonder I got this old after some of the stupid Sh*t I did. For those of you that have never been it and are preparing for the first time, of course train, train, and train as this increases your chances of being the last man standing, however, take every advantage of your equipment and those chances get higher
So yes, the flex tab conversion is mandatory on the older models if they are fighting weapon.
Also, I am not aware that Remington sells them to civilians. They are LEO restricted but the bums will sell you the parts separatly. You simply buy the breech bolt, slide and carrier and that is the flex tab conversion. When at the factory I asked why they did this and got some mess about the "Kit" is priced such to allow departments to upgrade their older 870's and are not needed on sporting models so not sold to civilians. Well it is sold just by parts at a much higher price. Bunch of bums if you ask me because many of these older guns are in the hands of civilians serving as defense weapons or doing double duty.