Is there any reason not to use a bottomless PF all the time? Do the spouts confer any advantage re shot quality?
None that I know of. When you start using it it can be unforgiving of poor distribution, so some avoid it because of this. I feel it's better to know about poor distribution so it can be remedied.
Is there a good scale that both measures shot weight (in g's) and times the shot? Preferably thin enough so it doesn't push the cup up too far.
There are several, I use a Brewista V2 which has as much functionality as any of them, being able to time manually as well as automatically from the moment espresso hits the cup.
A lot of attention is paid to tamping & distribution. But what about the moment when you tighten the PF into the group head? Seems to me if you do that w/ any force there's potential to make cracks in the puck & basically undo whatever good work you did w/ the grind & tamp.
Unless you're using a much bigger dose than recommended for the portafilter, there should be enough headspace above the pick to stop this from being an issue.
Hope this helps!