Yes absolutely. I often lose important email conversations in my inbox just because they get buried by other emails.
I hope that's what you mean but if not I would love to have the starred emails in the email inbox like the starred option in Gmail. And I believe it would be useful if you could star them in the deals view but especially in the email inbox itself.
For the deal module it's fine because more important conversations are usually further in the pipeline. But if you want to make it extra visual you could color the deals with starred emails like the rotten deals just with a different color.