You have had it for 9 years and never used it? Better late than never!

It is offroad traction control tuned for different types of terrain. Say, in sand you will want a traction control that is not very aggressive because if it is, you will get bogged down in a few yards. But in rock, you will want an aggressive traction control that stops a wheel before it has had the chance to actually spin.

And thus it works better than the one-size-fits-all A-trac that all 4Runners have. I practically never use A-trac though sometimes I do turn it on mostly for the heck of it. But it is not really useful or necessary on a Trail/TRD edition.

So, you are right, I use MTS on ROCK plus rear locker for ledges and big steps and I can climb with no spin at street psi. There are trails where you really don't need or want to use the rear locker. There, the ROCK setting on Multi-Terrain select works great.

I do not seek mud and have not had to go through mud even though I have done a lot in the 4R already so I cannot speak for how well it works in mud. I guess it works well.

But it is not a substitute for the locker. If you have your fronts on a steep rock and your rears in sand, you still want the rear locker or you have to air down, hoping to minimize spin. No spin with the locker and MTS even at street pressures on ledges where other trucks throw tantrums even if aired down.

Crawl control is a very aggressive mode of the same traction control system and it works extremely well for me when descending rough stuff and for going through rock gardens. I have never tried it uphill but I don't like rough descents and with no spotter I am happy to focus on steering while letting crawl control ensure that any hit I take would be at 0.6 mph. Its settings are 1-2-3-4-5 kmh.