Drip tips are an absolute necessity on any non-cartomizer atty, but I have never understood the idea of using them on a cartomizer.
First, with a carto, when topping off you really need to see the filler to get it right - you drip on the filler on one side and you can see when its about full. One drop too many and you have to dab it off w/ some TP to get the best vape. Vaping over full reduces performance until the overage is vaped away and also increases the chance of burning. If you have to take the tip on and off anyway, why use it at all.
Second, the bore on most drip tips is a bit on the narrow side, juice can sometimes not make it all the way through you get a bit of juice in mouth when you draw it out.
Third, sometimes the carto guts sit a little too high to take the drip tip.
Fourth, cartos just don't leak unless they are flooded in which case you need to dab it off anyway. You could leave the top totally off and not have leakage in the pocket. The only real reason for covering it is to prevent lint from getting in the carto.
Fifth, cartos just don't need it. Pop the rubber cap off and there is your drip tip. They don't get hot like other atties so you don't need the lip protection.
The only reason I can think of is if you just like the look of a drip tip. That's my biased two cents anyway.
+1 for just throwing a condom over the tip when putting it in your pocket. I mean, that's really the biggest advantage cartomizers have over other atties - they don't leak and you can put them in your pocket. Any other atty in the pocket and you are asking to wear your juice like a perfume.