I use Fast Ferments and just stick a plug in the airlock hole when I cold crash, the lid seal & plug hold what little vacuum is created during crashing. As for the bag thing, I've been using a supersized balloon of CO2 attached to the airlock when bottling for several years now. Probably accomplishes very little, but almost certainly doesn't hurt.
I guess it's a bigger deal for me than it is for others. I never like racking a beer and displacing with air. I much prefer a closed transfer system. The only time I am not closed transfering is when I am bottling and bottle conditioning and even with that I am considering using a CPBF to purge with Co2 first (not filling under pressure).
One of the big benefits is if you are racking and you accidentally screw up your process and bubbled get in the line -- that Co2 instead of air.
Another big bonus is the yeast cake is under Co2 instead of unfiltered air (unless you use a filter
FTR you can also purge your racking line if you have a "siphonless fermenter" by having a small amount of Co2 in your keg and attaching the line via a QDC with port "closed" -- the Co2 will purge the line pushing the air out that tiny hole that is on every single plastic spigot I have ever encountered (the reason for I have never completely fathomed).