I often do this. With loose leaf tea too. The quality of the result highly depends on the tea and whether you get the timing right. I know my microwave pretty well and can hit boiling or just before boiling by changing the time for a black vs a green tea.

When boiled appropriately, I can’t really tell the difference for most bagged teas, so maybe I’m just tea uncultured?

The earl grey loose leaf I have I actually like better when it’s kept boiling for longer (about 15 seconds of boiling), and the microwave allows me to easily do this.

The loose green tea I have changes its flavor a lot when heated for different amounts and to different temperatures. The microwave also let’s me easily control this in a way that I would struggle to with a kettle. I suppose I could add the tea afterwards and just get the water a bit hotter to compensate, but I’m lazy and I always forget about my tea in the microwave so it’s easier if it already has the leaves in it so I don’t have to re-steep

Maybe, who knows? Reheating tea though is absolutely foul. Worse than reheating coffee, somehow, and reheating coffee is pretty bad.