This is a minor comment, but a possible drawback would be that it might mask an artifact that would normally occur. For example, if source material is 44.1kHz, when edited in Shotcut, if you make a split on a clip on the timeline an artifact will occur due to a new audio resampler being created at the split. However, if the proxy has been normalized to 48kHz, then the artifact would not occur in the preview, but it would still occur in the export when not using the proxy.
This problem already kind of exists anyway if you export in 44.1kHz because Shotcut always previews in 48kHz.
In general, I would support normalizing proxy files to 48kHz to avoid sample rate conversion altogether when previewing in Shotcut.
For people who really care about possible artifacts due to sample rate conversion, they should be sophisticated enough to pre-normalize all their source files. We might want to add a sampling frequency option to the “Convert” dialog so advanced users can choose how their audio should be normalized. Might as well add channel layout as well.