Do any of you use speed and cadence sensors on your MTBs? I am not sure if they are needed since I can't imagine you are looking to stay at a certain cadence or worried about maintaining a certain speed over a track but curious to hear if anyone does.

I do.
I do a bit of XC MTB racing.
Can be nice to know the distance to the next pit stop.
Which gives you speed too whether you want it or not.
(Average) Speed can be helpful if you're trying to make a certain time.
Cadence is helpful to me since I have a tendency to mash instead of spinning, which my knees doesn't appreciate. The CC reminds me when I fall into bad habits.

GPS is more prone to dropping out with tree cover, especially at relatively low speeds. A speed sensor will give you more reliable data if you care.

I just use a GPS computer without sensors though.

Thanks for the input so far, I guess it isn’t a bad idea after all...

I use one. If not for anything else... maintenance schedule at mileage. I like XC as well as technical but I also swap out my mtb wheels for road wheels to ride with my wife. why? Because knobbies don't last long on pavement.
I hear a cadence of 60-80 rpms is ideal when climbing. We have some long climbs in the PNW.

I am not a racer or speed cadet, but does cadence sensors help when in going over rock gardens of other very slow obstacles. I know my gps will think i am not moving in some places, but I am working my ass off.

If you are using a garmin they have an option for Auto Pause under Training set it to only when stopped if it isn't already.

It will auto-pause on sharp corners if you don't have perfect reception.

I used to use the speed and cadence sensors while mountain biking. Now I still use my Garmin but without the sensors. I use the Garmin to track my ride knowing that there is too much going on during the ride to continually gander at my screen.

While riding you always have a change in elevation, direction, terrain, oncoming riders in singletrack, tire grip levels, gear changes etc.

I train HI and ride LOW! Summer MTB rides start out around 8,500 ft and go nowhere but up. Winter riding is so flat my road bike hardly ever needs gear change. I found that MTN bike got me into bad cadence habits - like very low cadence and early tired legs. So I finally added speed/cadence sensors to the MTN bike. At 78 I find spinning best no matter what or where I am riding.

I use nothing. I am a low-tech rider on a high-tech bike . OTOH, I totally get why some might find it interesting and useful to monitor and track data over time. Doing so can end up being a fun little hobby all in itself. It's all good, imho.