When to purge parts is one of the more important decisions one can make in P7 and we've been discussing it over on Discord lately, so I thought I'd copy some of the relevant points here for reference.
bugsniper: how do you leave behind damaged equipment?
b_sen: as in, you've purged to get it off you?
b_sen: either be very fast and outrun it, or fill all your slots with other things first
Kyzrati: another tactic is to position yourself over a group of new parts/stockpile that you'd rather have, since surviving purged parts will be forced out beyond those
b_sen: that is one way to fill all your slots with other things first, yes
Kyzrati: well you should almost never want to get rid of damaged parts simply because they're damaged unless you are intending to replace them immediately
b_sen: true
Kyzrati: so this is basically the main use case
Kyzrati: only other reason to purge might be if you're insanely overweight, in which case you purge then can maneuver to mostly avoid the heavy stuff and prefer propulsion etc while moving away
Kyzrati: (I mean "only other reason" other than if you simply find really cool stuff, of course 
)
b_sen: items move at speed 100
Kyzrati: yeah you can outrun them fairly easily as long as you don't get too close or aren't really slow
Kyzrati: "oh no! it's a crappy item coming for me, run!!!!"
Kyzrati: ah right, you can use doorways to block unwanted parts, too
Kyzrati: the main tip for purging is try not to rely on it
Kyzrati: you shouldn't actually need to do it all that much, it's mostly for emergencies and extreme cases
Kyzrati: it's much better to just preempt the need to purge by relying on good positioning relative to items in the first place
Kyzrati: all kinds of relevant tactics here, like specifically entering rooms through walls that are closer to the items that you want, rather than the doorway
GJ: I've started to purge a bit more during recent runs, or maybe just a more consistent amount. Some of my older runs had a lot of Purge Count 3 and some Purge Count 14, more recent runs are almost all Purge Count 8.
GJ: It's nice to purge whenever you can trade up in propulsion and integrity. Laserbots in particular can generate some attractive natural stockpiles where after you kill them there's an appropriate amount of legs and some cooling on the floor to just purge then and there.
GJ: Usually don't like purging if there's enough items around to fill all my slots then and there and no means of escaping some items by using doors or whatever
GJ: Partly because when I'm full on parts I trim good stockpiles by attracting bad parts or even some good parts if there's an excessive quantity of them (in positions where you'd attract too many of them trying to grab some specific item you want), expecting to revisit those stockpiles to grab a few items after I've purged somewhere(edited)
GJ: Typically that's what you want to pay attention to, you're usually purging to free up slots for good stuff, make sure you'll have enough propulsion post-purge to not be in a disaster situation, and try not to fill your slots before you reach the good parts you're going for.
GJ: Sometimes that can mean not killing enemies shooting at you while you move towards a good stockpile you've previously seen that's some distance away, as long as you're fast enough and the enemy count on the level has been reduced enough that you won't spiral into a catastrophe ignoring those bots until you fill your slots.
GJ: Because killing enemy bots usually deposits stuff on the floor close enough to you that you attract them
