I think that .io domains are popular for a few reasons, they are geeky "input/output" and they somewhat resemble a 10 (binary 2). Of course you mentioned the availability aspect which is huge. I personally own two.io (not sure what I'm going to do with that) so I may be biased :)
I think companies should stick with .com domains. They are the gold standard. That's just my opinion.
drop.io probably started the trend. They were technical finalists at south by south west in 2007.
The way I see it, if your product is technical then .io is fine. But if you eventually want to market to every tom dick and harry then you better get a .com. Another strategy is to get a .io until you can afford a .com. Also, if you can market your website to achieve a million users on .io domain, you can raise enough money to buy the .com. My 2 cents.