When AV used to be x1 to x0, 70 had a decent argument for PvP play.

Now that all BG's are x0 to x9, anyone who doesn't upgrade every expansion is the lowest level in all pvp brackets.

So you'd be better off at 59, 69 etc.
But with frozen experience (an in-game option), you only queue against others with frozen experience too.

And games don't start until there are a full number of players on both sides; not saying they will necessarily all click "enter battleground", but no one gets that window until the maximum number of players are queued for both sides.

There was a lot of content at 70th, prior to WotLK.
Staying that level for a while, doing the heroics, sunwell dailies etc.
Buying each toon an epic flyer, get tradeskills to 350+...
Lots of things to work at.