I remember in the 80s and 90s the term "dual purpose" was generally used for coach seated vehicles that were able to run bus routes. Alexander T and TE bodied Leopards, Tigers and Dorchester's in SBG territories for example.
The term seems to have fallen out of use, but coach seated buses at the same time have become more and more common - service bus B7RLEs, B9TLs, and more.
Is that because the bodies are seen as either bus (Wright Gemini for example) or coach (Plaxton Panther for example) clearly, and the 'in the middle' types were killed off?
Personally I miss them and think there are routes that would benefit from a middle option!
I think nowadays there is a greater distinction between a bus (with a low floor) and a coach (with luggage space below) - the only exception being the Plaxton Panther LE, with its bus-like front and coach rear.
Now I'm not totally familiar with the era of dual purpose Leyland Leopards, Bristol REs and such, but if they were designed to be equally adept at being a public service bus and a touring coach, I don't think it's possible to create such a vehicle now. The present day role of 'dual purpose' buses would probably be for bus routes of an interurban/limited stop nature where high-floor coaches are unsuitable, while also being able to tackle 'normal' routes when needed.
I think Stagecoach Gold nailed the modern day 'dual purpose' bus with Enviro 400 bodied Scanias featuring high-back seating. The Scanias feel more robust than the all-ADL version, with the added benefit of being just as at home plodding around on local routes as they are at being hammered down A roads on interurban services. Heaving said that, an older a bus becomes the more coach-like it can feel, now I'm seeing more Volvo B7TLs taking over the role of the typical school bus/coach from Olympians, with seat belts and carpet on the walls!
There doesn't seem to be many dual purpose candidates for modern single deckers though - midibuses like the E200 MMCs and Streetlites wouldn't be up to the job so an operator's best bet - as you say - is to put coach seats in something like a B7RLE that has a decent heavyweight chassis. I know operators like midis to reduce costs and whatnot, but I think there is still a small market for something along the lines of an Enviro 300 MMC with a Volvo/MAN/Scania chassis.