Biggest advantage is if you might need to use that smart list anywhere else, or if you would want to see who will be in the smart list without referencing the Schedule tab. Personally, I like being able to see/tweak who the send will include without going to the Schedule tab.
Bree, we wrestled with this when we first began using Marketo. First we created Smart Lists and then we thought "why duplicate the work?" and stopped.
Then we ran a couple of campaigns which impacted far many more leads than anticipated because of a simple and/or error. Another downside is that as our Smart Lists got more complex, it started taking longer and longer for our campaigns to execute, which threw us off for time-sensitive events.
Now what we do for emails campaigns (and I am talking ones targeted to promote specific events, such as trade shows and/or webcasts) is we use a smart campaign to populate a static list that we then use for a second out-bound email campaign.
Pro to this - you get accurate count of your outbound email flow before you begin, and your mails go out at exactly the time you want them to.
Con - 1. There are obviously extra steps to that must be taken. 2. It does leave you with a lot of "Assets" and Smart Campaigns which clutter up your Marketo instance (unless you’re diligent about taking the time to decommission everything.)