Is it that Tamsin is "good" for Bo or that Bo is good for Tamsin?
In point of fact, I think Bo is "good" for everyone.
She has a tight connection with the light fae through Dyson & Trick, with the humans through Lauren & Kenzi and with the dark fae through Tamsin & (?) Veck.
I think her power to affect them all is the key to her helping the fae, all of them, defeat the Wanderer.
As for Tamsin... she's kind of like the scary version of a fae Kenzi...
in that she's irreverent and can drive Bo crazy but can also make her laugh yet not worry Bo so much because Tamsin's a fae in a fae world, not a human in a fae world.
Of course, she's also a fae Kenzi
that succubus Bo wouldn't mind stealing chi from once in a while.