I imagine people often use io domains because it is hard to get a good domain name as a .com. I don't think it is being done for reasons that could be considered as "overkill".
Why not buy the .com too? Plaidio.com isn't taken, if a startup takes off 'io' doesn't work well anymore. I understand the io trend but I don't think that 'money.io' is ever going to be able to afford money.com.
