@TheGadfly here's my perspective as a small business employer.
I work really hard to make sure that employees in my small CPA have good work-life balance while making a good living. And
I would worry someone running a sideline business would sabotage that work-life balance. E.g., someone's working weekends, burning out, and then Monday am sure they show up but they're always just a little more rundown than they'd be otherwise.
FYI, working in a professional services firm may be a special situation. But to make this not about your situation or my situation but about other situations we'd encounter... or see.
Think about the doctor you or I go to who's got a sideline gig. Do we really think she or he is going to be as good and up-to-date and alert enough for the level of care we want? I always wonder. Sorry.
Or another maybe inflammatory example... but if you've got some superstar player on the favorite sports team you're rooting for, how smart does it seem if that person is in another sport or running some sideline business that you swear detracts from their on-field performance. That just seems to dangeruss to their quality of play.