Do it for your instructor... seriously... when I have been working with a student for all those classes, and they finally get to 50 dives and come in for that card, it is one of the coolest things ever! We get in this business to share our obsession with diving, and to see a diver go from being a little uncomfortable that first time he/she took off the mask in the pool to MSD... that is so cool for us!
A lot of shops also do something a little special. When I did my master scuba diver, the shop I did it with had a little plaque with all the Master Scuba Divers' names on it, and it was really neat when my name went on the plaque. Of course, then I looked at the plaque next to it, and suddenly I was enrolled in the divemaster course (then instructor, then MSTD, then IDC staff... one DM cert away from my Master Instructor ... oh my gosh! that devious s.o.b.!)
PADI does charge for the card, and only lets you use PADI specialties too (but you can use any aow and any rescue that it recognizes as equivalent). My dive shop ran a special that if you did the specialties with them, they would eat the card, and I have seen that in other shops as well.
Other agencies have different rules on it. For example, I think SDI/TDI lets you use other agencies' specialties (with some limitations) and is not charging shops for the Master Diver card (but is charging for all the specialties leading up to it).
So, yeah, the card is a marketing tool to get you to do more training.
But still, get it anyway... with an instructor you have been working with (not just any random instructor). The reason: it is a big accomplishment for your instructor, and he/she will get a warm feeling in his/her heart (not just his/her wallet) when he/she hands you the certificate.