I ended up doing that (taking sinner as a backstory but not for karma) because he left that life behind. It seemed abusive to take the karma and not pay the taxes.
You mean the things about "SINners with X SIN pay this amount of their gross income as taxes"? I don't think the PCs actually have to pay a percent of the money they make to the corp. That makes no sense.
Granted, that does also mean that if they were captured by the corp for any reason they could be put into jail for tax evasion, but isn't that the traditional "how we finally nailed this huge criminal" story?
About the SIN in general, you definitely would risk getting a hit team sent on you if you ever ran against your own corp, because as people have said, any kind of biometric evidence would immediately point to you. However, Corporate SINs aren't even shared with the
Global SIN Registry, and it's established that megacorps are very secretive with any amount of information. It actually means that if you have a Renraku Corporate SIN, and Ares Macrotechnology gets some blood flakes or something, they have a harder time matching it to you than if you have a National SIN, and even if they found out somehow that the runner who's biometrics they have worked for Renraku, you better believe they would have to pay out the ass for that information, because here's how it'd seem to Renraku:
"So you want access to our private SIN database in order to go after someone you suspect
might be affiliated with us, in order to stop them from causing problems and setbacks, while at the same time pointing the blame on us? That will require a check with a
lot of zeroes."
But the thing that makes it worth 25 points, in my opinion, is the reason why your character has become a shadowrunner. Someone with a Corporate SIN does not just become a runner for fun. From the moment they were born, they were sculpted to be a corporate drone. They were in a position many wageslaves wished they had, where their life was laid out for them already, where they'd almost automatically have more power and opportunity to advance through the corporate ladder as more or less a birthright. Whatever put them into the shadows is why they'll be hunted by their corp, not just
because they're a runner. And as others have mentioned, if other runners find out you've got a corp SIN, the best case scenario is that they see you as a massive liability, because hanging around you could lead to them being between a Renraku hit-squad and you. Even worse, those players might do as Michael mentioned-- Assume you're a spy, and possibly try to kill you. Or they might realize you're on the run... And you better bet Renraku is willing to pay big for your location and capture.
If I was GMing and a player wanted to pick that NQ, I'd have to make sure we discussed what exactly happened to throw this character out of the high life and into the gutter. That would set the tone for how often they may have to fight or run from their past, how other players might react to their past, and how (if at all) it could eventually be overcome. After all, buying off the NQ in the future is possible. The likely months-long story that would be the result of the character trying to burn their SIN, possibly fake their death, maybe eliminate or make a deal with the head executive who is looking for them (after all, unless they pissed of the CEO himself, it does come down to the small group who want him dead, possibly to get rid of loose ends, or because he may know something that could incriminate them), and then create a new identity would be pretty awesome.