It's an easy calc.
1. What are the current cloudbase/vis minima for Hebridean's VFR ops
2. How many times a year have those minima led to flights being cancelled
3. Given the terrain constraints at Oban, how many of those cancelled flights would have got in if there'd been an IAP?
Personally I think the answer to 3 is probably "none". Not least because the only possible straight-in approach is to 01, but the wind favours 19 most of the time, so circling approaches will be the order of the day and the circling minimum will be 1310 feet - higher than they can do without problems on a VFR flight.
There are plenty other questions too, e.g.:
4. Can the three runways meet or be upgraded to instrument runway standards?
5. Will the CAA approve instrument approaches to airfields with only an A/G operator rather than a FISO?
6. If the answer to 5 is no, will A&BC pay for part-time firemen on Coll and Colonsay to be trained as FISOs?