Actually, as a controller, this makes sense. Hopefully most of you know that ATC start to initiate SAR if you fail to call a departure ten minutes after you give a taxi call. So, in effect the pilot is saying that "I know it's close to the time you'll start bugging me, I'm still here & alive, just airborne, I'll give you a call in a tick". At least that's what I have seen it used for.
Agreed. I only really hear this when centre start calling for a departure report. The aircraft concerned has often been delayed by traffic and is heading down the runway. Letting them know you're alive and well and will call them as soon as you have your **** together seems like a good idea to me.
I once heard a QF B737 calling a five mile final at YPKA. "QFxxxx, 5 mile final runway 08 full-stop landing." I had to bite my tongue and didn't say, "so not doing a touch and go today then Qantas?"
"Pending clearance" doesn't achieve anything. If centre wants you to remain outside controlled airspace they still have to say it, and you still have to read it back. Just call climbing to same way you tell them you're tracking to a point within controlled airspace. You don't say "Tracking to YSSY pending clearance" do you?