I found that my flasks tend to make more of a mess than the individual serve packets (except when you keep a spent one in your jersey pocket when you get back in the car and it leaks onto your car seat fabric! Not that I'd know ANYthing about that! ).
My Gu flask (with a pop-up top like a water bottle) never seems to close completely, leak, and when I try to push it closed with my teeth, invariably pinches me in the lip. Ouch!
Maybe I have reject flasks. Maybe I have reject technique.
Frankly, I think the whole lot of 'em just aren't worth the effort.