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Unless it's a really big skimmer designed to be ran outside of a sump I wouldn't do it. It only takes one time for the skimmer to overflow and you'll wish you had it in the sump.
If room isn't a problem you could put the skimmer in a rubbermaid container (or something similar) with small pump hooked up to a water level switch that would turn the pump on if the water level in the container rises to a set point. This would give me piece of mind.
We have an MR3R and it runs externally. It is a monster though with 2 pumps (feed & recirc). If you were to use gravity feed (& no recirc) though you might be able to get away with an external skimmer without a pump at all. My skimmer is hooked up to a collection cup with a stop in it. I don't worry about it overflowing.
Depends a lot on the skimmer you get, I'd guess. Most people seem to run the ASM models in sump v. the MRC models that seem to be largely run external.
I agree with cayars, I had a skimmer leak all over the floor, luckily I was home and caught it in time before any damage, ever since then I prefer in sump.