I just added a hardtail to my MTB quiver, and I have no intention of riding it any softer than I do my full suspension bikes. When rallying hard through rock gardens, or coming off drops / jumps without good landings, you need a little extra piece of mind that you are not about to blow up your $$$ rear wheel. This liner (while admittedly is a pretty expensive pool noodle looking piece of foam) provides that piece of mind, and offers additional damping on the rear tire that air alone can't provide. Should you flat, you can also ride out on this thing and not have to walk home.
200% worth the extra cost for the item, especially if you are pushing the limits of low PSI on a hardtail. You spent a bunch of money on those wheels, you can probably swing another 100 to make sure you don't wreck them.
As Clayton mentioned, not difficult to install. A bit of a bear to get the tire bead seated fully when working around the liner, but by no means a show stopper. Probably took an extra 5-7 minutes vs set up of a normal tubeless tire.