All depends on use and needs. Having it a little longer is nice when using a bipod and a plate or wanting the bipod forward for some reason. I have more of mine like below on my match rifles but have one that goes to the end of the stock. Given the choice I would run them like below but in an AR it would depend on the use.

All the Arca. All of it.

Seriously, like Rob said, it depends on the application. My Comp rifles have full length plus the spigot mount. It's useful for tripods, plates, and bipods with Arca clamps. It useful because you can move the bipod or gamer plate anywhere on the chassis for stuff like shooting off of tires, etc.

My gas guns, even the precision ones, are more of a running and gunning gun. I use a small piece at the balance point to mount in my Anvil-30, and that's it. Maybe a gamer plate, but still near the balance point. A handful of aftermarket Arca sucks, so I don't want full length on a gas gun rail.