The main benefit is the flexibility of creating hashtags based on projects, people, phases of an arbitration, categories of projects, etc. The great benefit of custom mail hashtags is that it needs no plugin and no additional steps. Just type the hashtag in footer of emails and make it far more search friendly.
It's easy to mess this up if the hashtag usage is not consistent. Also, the recipients of your emails may find the hashtags to be a curiosity. Best to keep a 'Note' or a sticky to keep hashtag usage consistent across all mails.
I had thought of filing a patent on this, but felt that getting over a 35 USC 101 or 103 would be hard :-)
