No, it just prevents you from using an add-on when you don't have Blizz-ABS installed. Imagine somebody with an IQ of room temperature installs "Kill Count BABS Add On" without putting Blizz-ABS above it. In fact the person doesn't even use Blizz-ABS. And suddenly it all crashes. And the person is like "WTF" and wastes our time by posting about a problem caused by his/her own stupidity. This way the person will at least get a slat into his/her face saying "YOU NEED BLIZZ-ABS TO USE THIS SCRIPT, DUMBASS!".
And in fact it should be:
if !$BlizzABS || BlizzABS::VERSION < 2.51
raise 'Blizz-ABS v2.51 or higher not found! RMXP will now close.'
end
The SDK is not a bad concept, it was just terribly realized. If I had made the SDK, it would be compatible and people wouldn't hate it. :/