Medicineman, I get the sense that you (and those you represent) play as though the Sixth World were a static thing, filled with isolated elements - that what you have and are and do are seperated from other people. I'm sure that works for you, but Shadowrun as a game is intended to be interlocking, mutable and mutating, with interactions and changes depending on who the characters are and what they do. In this case, we're talking about a SIN - a Full Corporate SIN.
While I don't agree with firebug that Full Corporate SINners universally reside in the highest strata of society (because I believe, and I think rightly, that there are Full Corporate SINners who are janitors and dishwashers - who else is going to be trusted enough to do the scutwork in the high-security corporate HQ than someone who's spent much of their life inside the corp?) I think you underestimate the general connectivity - and degree of interest - of people in the world of Shadowrun. It doesn't take someone to scan your SIN for them to find out about you; lift a hair or a spit sample, and you have DNA. Get a glass, and you have a fingerprint. Strike up a conversation, and you have a voiceprint. Then go to someone who knows someone - your local 'get-things' guy, who may be two steps below a real fixer, but he does know a real fixer - and turn things over. Your teammates might be interested enough to do this; in my games, your fixer definitely is.
'Cause see, scanning a broadcasting SIN isn't the only way to check someone out. You can feed the above information - DNA scan, fingerprint, voiceprint, a retinal scan if you can get it - into a SIN query, and see what comes out the other end. You'll have false positives, sure - and you might get every single fake SIN the guy has - but only a few SINs are going to pop up on all of those scans, and only one is likely to look Just Like The Guy In 6-D. So now your fixer / fixer wannabe / neighbor / teammate knows. And maybe he tells someone, or tucks the info away in a file; maybe the word gets out. Maybe you piss him off, and he lets the word get out. Then again, maybe he doesn't - but when (not if, but when) he does, your life expectancy might be measured in hours instead of years.
Add into this the fact that if you're in the shadows and you have a Corporate SIN, any time a query actually returns your SIN, it's going to send up red flags to corporate security. "Who asked? Where are they? How close a match to the SIN of Wanted-For-Questioning Man is the information? Did they include an image? Most importantly, how soon can we get someone down there to take a look?" If word gets around that a Shiawase snoop is asking a few pointed questions about the lithe girl in 5-B, people are going to start paying attention. And when - again, not if, but when - that information gets attached to You, Yes, You There Behind the Woodshed, Stand Still Lassie, not only is your ex-corp going to be gunning for you, but you won't be able to find a safe haven in the shadows for less than double the going price.
So essentially, a Full Corporate SIN means you get it from both sides. Being a known criminal in the shadows? Hell, that's just par for the course ...