You can only shut down the outers if the APU and its generator are working (since only Nos 1 and 4 engines on the BAe146 are fitted with generators).
There is of course a very slight risk that should the APU generator fail after shutting down the outers then it could be mildly embarrassing.
In 20 years I have only ever heard of this happening twice.
Saving fuel is a much bigger issue.