That still happens today i.e. bf 3 and 4
In bf 3, iirc the lower the resolution, the less visual recoil there is and in these games generally everything on low/off except mesh will give you an advantage i.e. you don't have as much crap floating around, less dense bush areas etc. so it is easier to spot enemies. Also, for people using MSAA and/or vsync, they are just adding input lag thus giving a massive disadvantage to themselves.
Personally I don't have an issue with this, I do it myself when it comes to FPS MP games, more so to maintain a constant 120FPS though as I hate FPS fluctuations + if I drop below that, screen tearing is more noticeable. If I wanted shiny graphics and an immersive experience, I would be playing single player
The same could be said for people with 144HZ screens, better gaming mice, better sound equipment (so important this), better network connections etc. etc. Anyone using any one or all of these things will instantly have an advantage over someone using a bog standard 60HZ screen with high input lag, a crappy £5-10 mouse and/or a basic headset.
Same even goes for these 21.9 screens, you get more FOV at the sides than a 16.9 screen:
This is even more advantageous when it comes to helis and jets too.
EDIT:
It is even worse for CS:GO too.
EDIT:
Even some asus ROG monitors come built in with HIP fire crosshairs, a lot of FPS MP gamers use blu-tack on their screen too.