Final CRZ level may make a nice difference, so I'd say they are just trying to make the best out of the offer.
The road is long from planning (FPL submitted) to push-back. It's short from push-back to FL200, but tricky none the less.
Although the machine would tell you the optimum FL based on the forecast winds aloft you'd entered while on ground, you have acual wind data when airborne. Also, passing over the storm's incuses (geez, Cu incus, IŽd call it "anvil", but am not sure if this really works in English, sorry) may be a treat for pax comfort. One may even attempt to run step-climb calculation on the computer.
The CRM surely plays the biggest part too. Just give it for a tought for a while: skipper is the pilot flying and F/O is on the phone and you come with the question, ideas start to fly from one side to another. Or vice versa - even better.
Only well coordinated crew in favourable conditions (wx AND ops) will pass the message right away, yet there are many. My guess is 8 out of 10. We bastards learn to take advantage fast
. If you had asked about the "planned" FL, they'd all know instantly. There's no fun in flying planned flight levels.
Also I feel it is imortant not to leave controllers waiting, silence for many reasons is not an answer. My second guess would be, that vast majority of those taking their time come back within 20 seconds. How close do I get?