You might want small-base dies in the following circumstances:
1. You want your large stash of reloads to work for sure in any AR you might ever own or shoot, not just the one you have now. The same reason factory ammo is more reliable than reloads in all guns in good condition. Factory ammo cartridge bases are smaller still....except wolf, and some blue box Federals....you know the quality stuff.
2. You own an AR, that is designed for commercial (.308, .223) rather than military, and it's designed with a tight chamber where accuracy is first and reliability is second. Varmint or hunting guns come to mind. My Remington R25 .308 is an example....hell it won't dependably shoot Wolf or even Federal Blue Box. But it runs most factory, (exceptions noted above), and my small-base sized reloads like water running downhill.
If you happen to use Dillon bottleneck sizers, you already have small-based dies...they don't make any other kind. I've documented that on this forum before, from Dillon. Go ahead, ask them. Why do you suppose they do that? I'm guessing its to prevent Dillon sizers from causing heartburn in their customers and their customer service reps.
Talking about "conventional wisdom" that isn't: Insinuating that all folks who happen to have AR's that don't feed conventional FL sized reloads well are stupid and can't set sizer dies correctly is flat uncalled for. Once you set the die to cam-over as per most instruction sheets....which is about as hard as buttering bread, tell me, what magic do you have to make bases smaller, so they will chamber without a hammer.
I had one guy on another forum say to grind off your shell plate a little. That's a great idea
....then you can bump the shoulder back even further, when a conventional sizer, cammed over, usually bumps it back too much already. Your gauges say you're bumping too much? ...get a small-base and back it out a hair.
Then there's the magazine writer "gurus" who insist small-based sizers work the brass too much. Think a .001 or .002" squeeze at the base is worse than bumping the shoulder back .005" when its not necessary? Sure it is. Besides, the havoc the AR action does to brass will make it unusable long before squeezing the base will.
To the O.P.: The new "AR" die sets from RCBS also have another ace, besides the sizers that make your reloads as reliable as factory........You get a taper-crimp seater.....which is, my friend, a good thing.