Remember the huge mess years ago, when Dish switched guide data providers (for the satellite-delivered guide info) and stopped using Gracenote? Even if the title of the show, season number and episode number (the data that is actually visible to the user) seemed to remain unchanged, the meta data behind the scenes (what the DVR actually uses to search for and record shows) was different from what it had been before. That caused all kinds of problems with timers that had been created before the guide provider switchover (and even with new timers that were created during the guide provider transition) and the only solution was to completely delete the old timers and start over with all new timers. Then, repeat that process the next time the timers screwed up.
The above concern may not be a problem if the guide provider that the station uses for the PSIP happens to be the same guide data provider that Dish uses. However, there is no guarantee of that being the case, nor even of all OTA stations using the same PSIP guide data provider as the other OTA stations. This can lead to all kinds of variations in the meta data that would not be visible to the user, but would still break any timers that you attempt to create. In short, it would be another huge mess.