A club benefit that has gone away since 9/11 was having a nice place to wait for incoming passengers, particularly on delayed flights. Hopefully, we will get back to a position of having the option to meet people in the clubs even if we aren't flying that day. The only way I know of now to meet people is to buy a fully refundable e-ticket on the day you want to meet someone, use the receipt to go through security, meet your arriving guest, then get a refund for the ticket. A hassle to say the least. So factor in this benefit loss against the lifetime price (which didn't go down) when you decide to opt for a lifetime membership.
Another item to consider is whether or not the airline will continue to offer lifetime memberships. I bought a lifetime membership in the Admirals Club (American) many years ago, and now lifetime memberships are not available. Maybe NorthWest will cease to offer lifetime memberships in the coming years. Then you may regret not spending the money when you had the chance.
So there are pros and cons, like any other decision.