1) The plant will stop working on producing resin and concentrate on making seeds when pollinated...by a hermie or otherwise.
2) A hermie that seeds itself, produces seeds that will carry on the hermie trait = worthless
3) Even a few seeds in your bud makes it taste bad
4) A hermie will pollinate your entire garden. Most of the seeds from the other plants will carry the hermie trait also (in my experience)
I could think of more reasons not to keep a hermie, but I will just suggest harvesting hermies as soon as they are detected and putting them in the hash pile or something.
You can still get nice bud off late hermies, if you chop when the bananas show up....but nobody wants the seeds off a plant that goes herm, or cuttings....so just get it out of the gene pool please.