Dumbarton is not the best of places for a coach transfer as there isn't a car park at all, although at that time the street outside will be pretty empty. It doesn't give much time for boarding though as the first services to Balloch and Helensburgh are not far behind it. Using platform 3 isn't an option either as it will be occupied as well. The waiting room/booking office are also on the other platform so not much use either.
In that position I doubt any passengers will be caring what facilities Dumbarton has. Some of them will be prematurely thankful they're finally getting on a train only to be massively disappointed, some of them will wonder why they couldn't have stayed on the coach.
All of them will be very annoyed.