I don't know why you asked the question, but I know why I was hoping for a response from the older guys.
Guess I'll double check the Man Cave in the meantime.
Good question. Like I said, most don't seem to care who the original purchaser is. With any other product, the manufacturer will tell you to go to hell.
Do you even know what a "registration card" is, in this context?
It's got nothing to do with gun registration, and "registration cards" aren't limited to firearms - almost every higher-dollar product comes with them.
Perhaps the OP has caused confusion by not calling them their actual name: "Product registration cards".
These are used by manufacturers for data collection, and in the case of you needing warranty work on said product, it has the date of purchase on it.
Mostly I do send them in, especially when there is some incentive like a free knife or range bag or mag as kurt suggested. It has nothing to do with the actual warranty, at least for those manufacturers I've dealt with. Send them in or not, the warranty is as it is stated, and all most companies ask for is proof of purchase. Some don't even ask that.
They use the information to build data bases they can use in determining marketing and product line. And I don't care if I get promotional emails.