First- just to clarify -- neither JobSlinger nor JobSlinger Plus require W-9 verification. Also, your W-9 information is not submitted to us -- it is securely stored with us as a neutral third party, in accordance with IRS specifications. From our perspective, IC Pro registration is purely voluntary.
The mystery shopping companies that are part of the IC Pro Coalition want shoppers to register as IC Pros. They don't use this information to report your income (since many mystery shoppers do not exceed the earnings threshold for income reporting), but as documentation of your independent contractor status, in case they are ever audited by the IRS. Authorized representatives of SASSIE companies that you work for (and have connected to on JobSlinger or JobSlinger Plus) can access this information if the government requires them to produce a W-9 form for you. Basically, if you become an IC Pro, the same people who are authorized to see your information if you submitted a paper form, can use IC Pro to get the same information electronically from a secure server.