The documentation says it clearly:
Using a mail queue is recommended for production sites.
I was not aware of this feature at all and actually we even built a plone add-on to take care of that (sending emails asynchronously).
Now that I've read about it I'm wondering if it's something that can be still recommended, or if I'm better off with our custom solution...
mailhost queue in MailHost tool is a recent feature (I think it has been added on Plone 4.3) so I think it's still a good way to go.
I used it in lot of situations where Plone still send internal newsletter and it worked. This prevented me to rely on external services or add-ons that change MailHost behavior.
I had issues only once, where I was forced to restart instances because of sending errors, but I never reproduced the issue.