That's a great point.
Considering non-technical reasons, once Sprint gets the SDC service working on 1x and roaming, the need for Direct Talk will also diminish. DT itself was a clever, yet somewhat clumsily implemented solution for the large number of holes in iDEN coverage.
The 'clumsily implemented' statement refers to the fact that users must manually activate and deactivate DT, which is likely related to what AJ stated about the possible use of RF components for both iDEN and ISM modes.
In fact, when looking at the FCC docs for the i355 and i335 (and likely other DT capable handsets), Motorola clearly states that only one mode can be operational at a time. It's not a huge deal, but, forget to turn off DT and you won't be receiving any communications, nor will you know when network service is available again because only one mode can be active at a time. It's also tough to coordinate group activation of DT.
Interesting - that's the first I've heard of DT being used in such a way.