I would suggest most 5* luxury hotels don't get much in the way of bookings from coach tour groups. Most Coach tour groups tend to stop at 3* hotels, occaisionally 4*, and on very rare occaisions a big 5*. I've seen the odd cruise ship group stay at the Royal Orchid Sheraton and Shangri La.

As for the Oriental attracting a more formal / high-level type:
(i) I personally find the atmosphere more formal at The Pen

(ii) In the UK The Oriental is heavilly marketed towards the holiday maker in travel brochures. The travel agent Kuoni I think used to send more people to Thailand than any other UK agent and The Oriental always features heavily in their brochure. Whilst the prices for The Oriental are high, they are really no more expensive than what a UK agent would charge for a very average 3 star hotel in say New York (Virgin is currently advertising the Sheraton Manhatten at £250 a night, 5* start at a whopping £500) and therefore I would suggest your perception that The Oriental's guests are more high level may be a bit off when it comes to European travellers - many of us perceive Thailand as a country where you can upgrade your usual travel standards.

(iii) This may have more to do with how one perceives themselves. I.e. - "If I stay here, the clientelle has to be good"