I like the idea of the "treasure hunt" style!
Why would any of the high class regulars want to spend their matrix time in sewers?

1. Their trusted novacoke dealer only sells/takes orders from the sewer drop box.
2. Hottest neo-hipster Blitz-clubbers posts the next location they're going to play/dance at "down" there.
3. Someone, or something, is leaving snuff-btl recordings there.
4. There are backdoors to sites hosting insider trading information (how to's, what to buy's, and/or who's buying what's).
5. That's where their child's e-ghost can be found.
6. NERPS
7. It's where they shop for all their human trafficking needs.
8. They're selling national/corporate secrets to other nations/corporations there.
9. They realized it's where their grandpa's old MySpace account would be and as blackmail it's sure to get them back in the will.
10. They've set up a tap on a rivals datastream there.
11. They're looking to hire shadowrunners with a specialty in Matrix runs.
12. They're looking to find, in order to kill, torture, eat, shadowrunners whose specialty in Matrix runs cost them dearly.
13. All the best pre-Goblinization fetish porn can be found there.
14. Genuine interest in all things Matrix.
15. Got sidetracked then lost while trying to order vintage TMNT paraphernalia.
16. It's where they've set up the betting on their illegal gladiatorial-style blood-matches.