^The downsides are extra connections (more points of possible failure/leakage) and the slight lowering of the oil filter, which may or may not be a concern.
Personally, I like using the galley plugs on the block and there's no reason to keep the oil pressure dummy light sensor, but it's an easy spot to get to for an aftermarket sender.
I found that the rear galley plug is a royal PITA to get to on the newer cars. My 05 wrx was pretty easy, but I had to remove the throttle body on my STi just to get enough room to take more crap out of the way. The only reason I got it done was sheer force of will, but I nearly gave up a few times and twisted the crap out of my allen wrench.
Then there's the whole debate on where it's best to read oil temps: through the galley plugs, the pan or the filter adapter...