I believe the SGS3 operates the internal partition in the same way, that's how stock ICS does it on a Nexus device too... Wifi dropping while asleep happens largely at night for you doesnt it? Its a power saving mode HTC put in, there's a fix available for root users, rather crappy feature to have with no user control (specially when Wifi can save power!).
It happens seemingly when the phone is not used for a while. I don't know how long- 30 min? an hour?
Yes, it is very crappy that HTC would do that, when there is a setting addressing this and they simply ignore it and do whatever they want. I suspect it is part of the reason for such incredible battery life- just break a core functionality.