I used to scan everything, even asteroid fields/belts.....took sooooooooooo long and quickly became mind numbingly boring that I started to pick and choose what I scan, even in undiscovered systems...just can't be naffed to scan the lot of them. Now if the scanning process was quicker then yeah. But at the mo, waiting for that bleep bleep bleep bleep bleep noise to finish is really ...well, crap!
If they speed up the scanning process, increase payouts for gas giants and have a payout for asteroid then things would be a LOT better for explorers, I'd probably spend longer in each system scanning everything again, but the process wouldn't be such a (I hate using the word but I have to ) grind. It's just NOT fun watching that spinny scan icon animation thing and waiting and waiting and waiting for it to finish.
I'd rather see scanning become a more interesting and interactive thing than see it become quicker. Not sure how it could be made interactive without becoming a stupid, repetitive mini-game, though.