Tango,
This is also true for American (at least if you are on an international itinerary). Plus once you hit 2mm you theoretically have lifetime access to the Admiral's Clubs when flying OW on internationa trips.
However, I still believe that it is cheap of American not to honor the TWA cards, especially now that you can't buy them any more. As mentioned above, I do not believe that there is a single case where the acquiring airline has not honored these memberships.
If they don't accept them, I feel particularly sorry for those folks that bought a lifetime TWA card AFTER the TW/AA announcement was made and before they cut off the purchases.
FF