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. Their bases are smaller still....except wolf, and some blue box Federals.
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 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 my small-base sized reloads like water down a hill.
You already have small-based dies if you have any bottleneck Dillon Dies...they don't make any other kind. I've documented that on this forum before, from Dillon.
I'd take up m&p45acp10+1 on his offer, but I already have both small-based and regular dies in both AR's most popular calibers.