Perhaps re-formating is not the correct term, others use re-coding. The end effect is you are re-codifying the clip and therefore you will loose some quality.
The big compliant of mine is even if you same as .wmv again it takes more space, from 30 to as much as 300% more. So you take a 100 meg clip and cut 30% and end up with a larger file.
If it is cut, bad parts taken out, and not re-coded or re-formated you end up with the same quality as original, and just shorter.