So, I've been looking into why Office, and therefore Outlook, doesn't offer symbols to ISV developers creating Office solutions in the same way that Windows symbols are available to those writing software on Windows. It took way too much time and emails to get to the right person for this, but I finally did, and here's what I learned. We (i.e. some people in Office) are actively investigating the value of providing symbols, given the cost to publish them.

Cleaning up the .pdb files for public consumption is costly. The first challenge is removing language that isn't appropriate for public consumption. Additionally, symbols in code from third party vendors need to be excluded. ISV developers need to be able to step through code and see Office symbols but not symbols from third party code - this can take a lot of work.

Also, up to now there has not been a call from the Office development community for symbols, so no percieved need exists. Even if there was a perceived need, it would need to fit into the Office long term strategy for it to happen. Though I personally would love to see all of Office and Outlook specifically become a better platform for productivity tools, I recognize that others may know better than I if that's the right way to go.
However, the right people are looking at the problem, and I've volunteered my help if the work ever does get done.