Twice this week users asked me how to get a "non-click-to-run version of Office 2013" because the addins they need won't support click-to-run installations. Unfortunately for these users, an installed version is available only with volume licenses typically available to large corporations. Smaller companies and home users are limited to click-to-run. So, pretty much any company that refuses to support Office 2013 click-to-run is writing off a growing percentage of their potential user-base.
Honestly, it shouldn't matter if a user has click-to-run or a volume licensed MSI installation. Add-ins will work with the click-to-run installation (and will work with 64-bit), but the vendors may need to tweak their addins to support it.
I suspect the problem is either that the vendors don't want to go to the expense of upgrading their product to support Office 2013 or, more likely, they are familiar with Office 2010 click-to-run and decided that Office 2013 click-to-run is exactly the same. It's not, but unfortunately many people assume that Office 2013's click-to-run is as limited as Office2010's version.
Everything you knew about Office 2010 click-to-run applies only to Office 2010 click-to-run. Period. Don't assume that what worked (or more accurately, didn't work) with the Office 2010 version behaves the same way in Office 2013. Microsoft learned that the click-to-run Office applications needed to support VBA and add-ins or no one would want it - especially business customers. So Microsoft fixed the problems and for the most part, users won't notice a difference if Office 2013 is click to run or an MSI installation. It's time for developers to stop using click-to-run as an excuse to not support Outlook 2013.
What's new for Outlook 2013 developers - see the section entitled "Coexistence with previous Outlook versions"