I think along with all the other good reasons mentioned her and on LJ, there are some good reasons to hold clips depending on your audience. If you expect much of your audience to be unfamiliar with the source, a longer clip gives them more time to orient themselves and to gain some context, either or both of which can be useful. Just be sure that there is enough information in the extended clip to do that. Alternately if you expect your audience to know the source VERY well (this happens much less often than it used to with vids - there is just too much source out there these days and most folks are fannish butterflies to one extent or another) you can use extended clips for them to do some of the work of connecting the clip to things you don't picture (think of the first vid in this year's challenge show and how the still images made the audience do the work of the connecting things.) In either case, if you do hold a clip make sure you give folks enough direction of what to think about so they aren't making grocery lists while waiting for the next clip.