As a founder, the temptation is real, but anecdotally (anecdataly?) I'm so much more motivated on the work days I don't think the extra day matters. But, my company is not big enough to have enough sample data to prove it one way or the other.
Employees do take the extra free time into consideration when comparing offers, since we make offers that are competitive to a 40-hour offer.
Re: sneaky bit of work on the off day. We don't rely on it, or ask for it. We do have an on-call rotation that covers support and site issues on Fridays, but you're in that rotation once every 5 weeks or so.