i use 18490s exclusively. i dont even own any 18650s, or the extention cap.
the reason why i chose to go this route was purely monitary. its not that i dont want more battery life, because who cant use more battery life, right? the larger size is of slight concern, but its really more about the money. and now that i hear that the extention cap isnt flat, im really glad i dont have one. i knock my provari over enough as it is, i dont need the cap making it easier to fall over.
18650 + extention cap = $$$
18490 + 18490 = $
approximately the same amount of mAh for both scenarios, but the 18490 option is cheaper. sure, you have to go screw-screw-screw, slide batt, screw-screw-screw, but to me thats just fine. im religious about charging, so thats not an issue for me. i can see why others will chose the longer batt life. its just plain easier. but for me, i dont mind doing the extra work. different strokes for different folks. its nice that we have our options. who knows, maybe one day i'll get that extention cap (if its flat, or if i get a nice caddy)!