You pretty much have it figured out already. It is almost a gimmick - but, to a lot of people, all they want is regular cable plus HD programming plus DVD. So, they don't want a digital cable box or all the expenses associated with it.
So, the OTA (over the air) HD tuner built into the set allows them to get their HD channels for free, then they just use their regular cable box, or cable ready feature of the plasma for tuning. Add a DVD player and a lot of people are 100% satisfied. They can watch the game in HD on Monday night, then watch a movie with the kids on Saturday night.
The options are to just get a HD compatible monitor. This has no built in tuner, or no built in HD tuner. You can then hook whatever tuner you would like to up to the display and watch to your hearts content. Of course, you need to find a place to put the set top box - because set top doesn't really apply to plasmas. But, you have the latest technology and can upgrade it more easily.
It's a personal decision what you end up with. I went without the built in tuner and saved some cash as I knew I was going to use HD cable w/DVR.