I also only use a separate account for open source work for my employer. There are multiple benefits:
* My employer has no say in my private account. They can't force my to change my private bio or profile name, for example
* It's clear which code I authored as a part of my employment and which I did on my own after hours (extremely important for me).
* I work in IT security do I should give an example :). Some projects my company hosts on GH are not public (yes), or some projects I have access to because of my work. I shouldn't touch them after I leave my current company, or access them from private devices.
I don't own that code so I don't associate it with personal accounts
But I've never done open source work for an employer either