Question

what are the advantages of having only one si unit for pressure???

Answer 1

upside is international comparison of data for example.. there is lots of positive sides and only few often cost related downsides ie measuring instruments calibrated to non si units....

Answer 2

You know how it's always confusing how Americans use Farenheit and Canadians se Celcius, so we can never understand each other? (Or kilos vs pounds, metres vs feet, etc)

So by having one SI unit, we would avoid this kind of confusion internationally.