I was wondering that myself, but best answer i got....from my own experience.
Pros:
-fresher flavor, it drips straight on then toots your mouth. (could be a mental thing)
-no tank means smaller/shorter set up, more compact in your pocket. You hit the club or walk around without a 7-in PV sticking out
-I use an Igo-W, 10 drops last me a whole session. Session as in a typical analog smoke break. The better dripper atties out there have juice well, so you can over drip, it'll collect on the bottom, then your silica wicks back up. Kinda like a half-open tank.
Cons:
-gotta carry your bottle, another thing to remember. Adds to the pocket list which already include your cell phone, car keys, work keys, batteries, pens, etc.
-Must leave PV standing or juice will leak out.
-with dual/quad coil becoming norm, your batteries tend to run out quicker. I carry 3 18350s daily. At home I just use a phat 3000mAh Samsung IMR