Hi, I found something interesting. As you may know starcoins are worth $0.01 each so when you have a thousand you can get back 10 bucks.
A better use (at least at .ES) arises through exploration of the tourney lobby the last days:
There is a tourney beginning 21:45 CET which entrance is 5 starcoins. If you make it to the payments, they award you with (more or less) $0.35 which is 7x the rackeback you get through the vipstore. Even more, If you get to the bubble just 4-5 times in a month you are profiting from this. Also Prize pool is great with $85 for the winner.
TL;DR. If you spend 150 starcoins monthly on the daily VIP bronzestars+ tourney is easy to get profit from it while training MTT skills.