Although I value sim time, I do also feel that nothing replaces the real thing - the realization that "I can't just pause this" causes a whole different set of pressures which are important to practice responding to. Plus, although a CFI can try to simulate ATC when in the sim, it isn't the same as the real thing, complete with garbled transmissions, blocked calls, last-minute changes, etc.
If an AATD can be used for an IPC, it is possible that someone could maintain their IFR currency for _years_ without even getting in an actual airplane. That seems like a problem.
However, now having written that, I realize that as it is currently, as long as you don't go more than 6 months (and therefore require an IPC), you could do exactly the same thing - fly a few AATD approaches every few months and maintain currency without ever getting in an airplane.