Hmm, both really, but more in the sense that managing several non-gmail addresses in the platform gets problematic.

In my client, for instance, I have four different corporate email addresses I have to keep track of during the day. In gmail you can only really see the emails from one of the accounts at a time (you need to interact with the top-level menu to see what other accounts are doing), where I need to be able to monitor all four at once. Using Apple Mail (and sometimes Thunderbird on my PC) makes for a better workflow.

Though personally I view a @gmail address as being personal and not professional. (i.e. If I'm contacting XYZbusiness, I want to see a @XYZbusiness.com email address to add a level of reassurance that I'm dealing with a legitimate entity). However I currently use a gmail address for my freelance stuff. One day I need to re-register my old personal domain again though, lol.