I'm not in favor of turning off self-voting. Holders of SP must have ways to profit from their SP while not powering down. If you restrict those ways too much there will be adverse consequences.
But two things come to mind as to why buying STEEM makes sense even without the possibility of self-voting:
If you don't power up STEEM, you can still buy it for speculative reasons, which is why it has mainly been bought in the past and why it continues to be bought to this day.
Also, if powering up in order to be able to self-vote makes sense, then delegating to bidbots makes sense, too, which requires powering up.