Me, I'd like knowing as a lifetime member that I'd made an investment in the game and the company that created it. It's a way to ensure not only that the WoW content keeps coming but that Blizzard itself remains healthy. A lifetime membership would cement my emotional attachment to the game while at the same time relieving the stress I sometimes feel to log on "just to get my money's worth." During times when I'm especially busy or even disinterested in the current content, as a lifetime member, I'd feel less pressured to log in simply to avoid wasting my regular subscription fee.
Of course, this late in the game's life cycle, there's always a risk that you might fall out of love with WoW or fall in love with another game after you'd already paid for a lifetime membership. What if all your friends and guildmates left? Heck, what if WoW became free-to-play, leaving you with adjusted perks you didn't particularly appreciate?
Would you take advantage of a lifetime membership to WoW if it were ever offered? What benefits would capture your interest? And how much would you be willing to pay for a lifetime subscription to World of Warcraft?