Aside from aesthetics, it depends mostly on your application. You can't get any closer to your work than with a choil--heck, you can even get over the work. I especially like ESEE choils, because they provide a comfortable surface area for the finger. Another thing I like about a choil is that having the guard between my index and middle fingers gives me firmer axial control over the blade. That, however, can be replicated with the right handle design--as is done on many folders (e.g., Kershaw Skyline). Further, it gives me greater leverage over the blade tip--where the protrusion under my index finger acts as a lever arm.
As mentioned, choils can snag--possibly with very negative consequences (e.g., bushwacking). To avoid snagging, however, you can smooth the transition from choil groove to blade edge to eliminate snagging, so I don't personally think that's a reason not to have a choil.
The best reason NOT to have a choil, in my opinion, is to retain blade edge when you want your finger protected by the guard. For example, a combat knife used for thrusting probably ought not to have a choil. Also, I didn't want my young daughters to expose their index fingers while cutting, so I got them choiless knives (Fallkniven F1 and Cold Steel Mini Hunter).
Generally, I don't want a choil, but I do like one on my ESEE-6, because it does broaden the usefulness of that knife--can't say I usually like it on my 3 or 4, though.
It's pretty obvious whether a choil is useful or not to anyone who actually uses their knives for real applications.
