They don't keep it off and maybe it retains it, the one benefit from them is that it protects the shafts from getting pitted if you run at a track with rocks & gritty dirt getting thrown at them.

But they also take away from the performance of the shock.

I have had ti-ni shafts for about 6 months and they are fine without boots. Most of my driving is indoor but once a month I will make it to an outdoor track. Maybe because westcoast rc is such a clean track, maybe that will be different for you.
My guess is that shock shafts will get bent and need replacing before they would start showing any pitting.
Pitting will mess up the shock shaft o ring causing them to leak and perform poorly.