Can any body comment on allowing "one" signer with multiple signatures (all the same name, but different styles) to sign one signature card with all the different possible signatures?

The purpose of the signature on a signature card is twofold: 1) obtain agreement with contract terms and 2) provide a signature for verification. While the customer may choose his own signature; e.g. R.E. Lee vs. Robert E. Lee, it does not make a lot of sense to provide multiple versions on a single card. Whether you allow it is probably a function of tolerance rather than legality.

But when you have a client who claims to sign the exact same name, but signs several (some time up to 10 different ones) dramatically different ways, is it adviseable to sign the signature with all the signatures? And how will that hold in court if the document is ever used in that forum?

Customers have more imagination than the legislature does and "the law" does not answer every question. However, judges generally have strong personal opinions. If you paid a forgery the judge may say it was the customer's fault for denying you the opportunity to have a single signature for comparison. Alternatively, the judge may say it's the bank's fault because the bank catered to a silly request.

I would lean toward the second analysis - you are agreeing to verify that the signature on the check matches one of several alternatives.