The Derby - Crewe service in its own right doesn't really have great traffic potential - Stoke station only serves a small area of the wider 'Stoke on Trent' area, Crewe is usually a destination to access other, long distance services. Between Stoke and Derby there isn't anywhere significant served.
Considering almost every service is well used I would say it could have potential to grow. I do have to agree about Crewe and Stoke on Trent but the stations between Stoke and Derby are at times well used. I don't often go towards the Crewe end of the route. During peak times there can be up to 20 people from Tutbury to Derby and back in the evening and even just as many in the opposite direction and I have seen this much outside of peaks as well at times. Alton towers and Uttoxeter racecourse also helps as the 1442 the other Friday was full and standing at the point of almost leaving people behind with passengers going to Alton towers.
There's good connections at both ends of the route for Scotland services. Crewe has around 6 to 10 minutes each direction and Derby is just over 15 minutes in each direction.At least until HS2 arrives, if it ever does. I've been following this thread for old times sake, as 40+ years ago my wife and I were users, since she came from Derby and we lived in Glasgow. It was a busy little train then, and I am surprised that it seems to have become poorer over the years. How good are the connections with the Birmingham to Scotland services? I would have thought that an extension to Nottingham would be obvious, but the biggest needis for Stoke to get out of the economic doldrums.