I'm a lifetime member because I.V. has given me great value over years. Also am a premium member because to me it makes sense to support the site. Finally and most of all, I'm a lifetime member because I'm a cheapskate with a $200 lifetime membership representing a much greater potential value than $76 per year annual or $108 per year monthly rates.
BTW, one need not be a lifetime member to support the site. Many premium members pay annually or monthly, and some others simply give a donation from time to time. So there are lots of ways that people support the site financially, without having bought into the lifetime membership offer.