In most cases, the -Def makes sense for a burst. The bullets spread out and are harder to avoid, but you're still unlikely to be hit with more than one.
In the case of guns like these though, it makes sense to have +DV instead. They fire two (or three) bullets not in a spread, but next to each other. It is not any harder to avoid than a single bullet, but if it hits it has significantly greater stopping power. That is the point of these guns, they are designed to treat multiple bullets as a single, more powerful projectile.
The biggest downside to doing it this way is that it adds an additional rule, which does make things more complicated, but it is limited in scope to a handful of very specific weapons, so that isn't too much of a problem. I don't know if it is unbalancing though.
Also, before any discussion gets started on the matter, I know this is not the actual rule in the book, it is just a house-rule suggestion. I will probably house-rule it this way in my own games, but I am not saying anyone else has to do it this way, or that any other way of doing it is "wrong".
