All you're saying is that likelihood of swatters committing additional crime is low, but there is still likelihood. There can be others taking advantage of this situation (or also mad at the couple), who are in fact willing to commit more serious crimes.

Serious crimes are generally a low percentage stat in society overall, which is part of the reason why it's so hard to judge "oh, this isn't serious". Most police encounters aren't, except the ones that are.