I heard because with lower res = lower amount of small mouse movement picked up. So pros use this for consistency but a downside is they cant get the very precise shots which require tiny mouse movement. Is this true?
and there are multiple posts/threads about how FPS players don't aim at PIXELS but in DEGREES (crosshair stays in the same spot no matter how fast you move your mouse.....)
On CRT monitors the reason is that they usually have higher refresh rate on lower resolutions. Also, 10 years ago, graphics was not so fast and it simply gave more fps.
Nowadays on LCD monitors and with fast video card it doesn't really make a lot of sense (but still may be useful on CRT). It is used because of mental inertia (or just retardedness - like, hey, pro player X plays on 800x600, so let me change my resolution and I will be pro too).
Since ischju plussed you i feel the need to elaborate for people who might read this and not understand.
95 fov is the same on 4:3, 16:9 and 16:10 in quake live (horizontal). There's no widescreen calculation required as the game already supports the resolutions, and even if there was, you'd just do the calculation and use 114.3423542525 or whatever instead of 95 (a guess, i cba). Either way, when set up correctly.. http://levelupyourgame.net/zronvc/files/aspect.gif
In other words, if people have a widescreen monitor and want to use a lower resolution they should use 1280x720 (on 16:9) or 1280x800 (on 16:10)... not 640x480, 800x600 or 1024x768 like many of the oldschool players use, those resolutions should only really be used if the monitor is 4:3 (unless you have some kind of inability to adapt to additional benefits).
The only real circumstance of which i can see the need to use 4:3 on a widescreen monitor is where a) the game just outright doesn't support it i.e. it doesn't work and/or stretches the image, or b) you can't hold stable 120/125fps with 16:9 or 16:10.