Some interesting comments and ideas on here. I agree that the I/J provide a useful link between Whipton Barton, Tesco and IKEA. I live not far from where the D sneaks into Pynes Hill by the back door so the J provides useful extra service back from the city centre. Whenever I use it, I am not the only person on the bus. When I suggested the buses divert through the new Hill Barton Vale housing I still intended that they would continue to Tesco etc.
It is a pity that when the new roundabout was installed in Hill Barton Road to serve the new housing, bus stops were not built at the same time. However, whilst Hill Barton Road may appear to be "fast" it is an ordinary urban road with a 30MPH speed limit. At one time, buses used to turn right from Hill Barton Road into Lower Hill Barton Road and, if the road sign is to be believed, they are still not prohibited from doing so. I accept that the police HQ would a source of passengers but anyone travelling there from the city would surely use the 9 or Green P+R. I cannot see that serving Digby & Sowton Station would increase usage.
I see from N&P that Stagecoach have registered a change of timetable and route for the D. I hope they are not going to muck it up.