I understand that, around 2010, ABP was modified to not count filter hits in Private Browsing to improve privacy. That's a logical decision. What I'd like to ask though, is to add an option to bring it back.
I'm very picky about my filter list and especially its performance. I manually (and using Element Hiding Helper) make my own filters. A lot of them are for sites I only go to in Private Browsing (image hosts and stuff, I do a lot of things in Private Browsing because I also automatically have Tor in it), so their counters stay at 0 at all times. Now, of course, filters expire after some time: I stop going to the sites, the site changes and the filter stops working, etc. The hit counters are the best way to find filters that are not useful. With Private Browsing hits not adding to the counter, that makes a lot of my filters unmanageable.
Code-wise, I can't see it being a problem. It's just a boolean flag that's added as a condition to the private browsing check. For users it's no problem either since it could be off by default.