Here is one intron benefit: Cancer prevention aid. Introns can take
hits from chemical carcinogens. These hits might otherwise hit and
mutate an exon causing more damage. The more introns there are, the
fewer damaging mutations occur.
Earlier in evolutionary history, viruses (now 'smarter') might have also
been thwarted. I'm not claiming this affect is large but I strongly
believe the affect is real.
Junk DNA might have a similar function if the histones don't protect the
unexpressed DNA from carcinogens. If anyone knows whether the chemical
carcinogens can penetrate the histones, then please e-mail me.