Sometimes it just happens. I didn't do 3 of my secondaries, but mostly because when I received them, the instructions said that out-of-state acceptances were very few and far between. And since I'm no stellar applicant, I decided to forego applying. And yeah, laziness also has something to do with it.
I did end up doing 21 secondaries (MD and DO) overall, though. So hopefully I get some love, somewhere!
The few schools I did get interviews at I ended up liking a LOT, and I feel really fortunate for that. I'd be thrilled to go to one of them in particular. But I'm still waitlisted, so sigh..YES, I regret not filling a few more secondaries just to be safe (Albany, Case, and Pittsburgh in particular) I just got too tired and discouraged, because by December I hadn't gotten an interview yet--so I was seriously contemplating giving up and didn't end up filling out more secondaries.
I also regret not initially applying to more schools, like MCW and Miami. I didn't even know about these schools somehow. Serves me right for not getting a MSAR or some sort of guidance..or for not being a avid SDN reader before the app cycle. I also had no idea you could add more schools to your AMCAS until the deadlines for each respective school. So..you're definitely not alone.
Once the rejections started coming in I really lost hope and couldn't bring myself to finish a few of my secondaries. I truly thought that I was wasting my time (my stats are kinda sketchy) and I couldn't manufacture one more drop of optimism or hope.
I regret it now that I've received a couple interviews because I think I could've had a chance at those schools. Live and learn, I guess.
If I have to reapply next cycle, I'll definitely do a lot of things differently.
Duke's secondary came in way later than all of the other ones. It also had like six essays with no maximum number of words. I got an acceptance to another school shortly after I received the secondary, so I never bothered with it, which I don't regret.