1) Does anyone know why Shimano puts different rings on road triples than on road doubles? Doubles seem to come with 53/39 combos, while triples come with 52/42/30. This seems odd to me; why not just add a third ring and leave the other two the same? Is it a silly marketing thing, or is there a good technical reason for this?

So of course, I'd like to change the rings on a recently acquired Ultegra triple to 53/39/30 (or perhaps even a 28 small ring). My LBS tells me that the rings are actually different and that such a change would be
"difficult." So...

2) Is this true, or can I easily change my triple rings to double rings?

And question 3 of my two questions: if I have the long cage Ultegra derailleur, can I use an old (unused) 28 Shimano MTB ring I have kicking around, or is the small ring road specific?