IMO selling is a pretty good feature, even if it kind of changes the balance. At least zealots are now somewhat viable (they were extremely risky before - they ignore air, they get raped by reapers, hydras, marines in numbers, archons, ghosts, roach/queen, banes), but now you can just sell them if they become useless. Similar thing with lings (although to a lesser degree).
Mostly it is not worth selling units, because even that 30% loss is pretty significant. I met someone who started with a mass marine, i responded with mass colossus, then he went for BC. I already had a significant number of Stalkers as meat shields for the Colo, so i had no problem with the Cruisers, but he had a problem because my Colos just went on to kill their main building. His cruiser numbers never reached too much because he lagged behind on money due to sells.