1. If you're still using HTTP 1.x, sharding assets across origins lets the browser load them in parallel (if set up correctly). You can generally load just 6 assets in parallel per origin, and sharding is a way to get around that limit.
2. A library like jQuery is so popular, and is so often served from googles CDN, that chances are a user already has it in their local cache from when they downloaded it on some other site.
That said, yes - the downside is more surface area that might go down.
