To be honest I probably would as well now I think about it... But you can't deny that slapping one in an E38 and then supercharging it isn't at least a little bit cool and/or different.
I can understand why someone would do it.
If done well to the right car I'd probably have an SWB E38 S62 & manual swap over my E39 M5. The cars are very similar in architecture anyway and the weight is remarkably close. However, if I were to do a swap it'd be the S62 & manual gearbox into an E31 8 Series. Still reckon if my M5 is ever involved in an accident (which would more than likely write it off) I'd buy it back and do a drivetrain swap.