I have not found anyone who asks this question, but I believe that Rashi here teaches us an important point. If Rashi would say that one may burn Terumah Tehorah with Teme'ah after the seventh hour, because the Terumah is in any event "Holech l'Ibud" (destined to be lost), this would indicate that the Heter to burn the Terumah Tehorah is due to the fact that there is no rule of "Mishmeres" on this Terumah. Rashi teaches us that this is not the case. In fact, there is still a rule of Mishmeres on this Terumah. However, since the Terumah is not suitable for human consumption, it must now be burnt, and as such is similar to Terumah Teme'ah that must be burnt. Since Tum'ah wil not now change the status of the Terumah one may burn it with the Teme'ah. We are assuming that Rashi holds that "Mishmeres" precludes any destructive use of Terumah besides its consumption in the prescribed way (see Tosfos to 13a, DH v'Sorfin).

Dov Freedman