Hi mskay!
I think that the answer to that is best answered by answering THESE questions:
Can I get a discount at a Resort at my time of travel with the Disney Club? Is there a code available to me at the same time? Do I want to worry about it?
How much do I shop at TDS or at the World of Disney?
Do I already have the Entertainment Book?
It is by getting savings on these things where you will get SIGNIFICANT savings. But if there is NO DC discount at your time of travel, if you do not do much shopping at TDS and the WOD and if you already have the benefits of the Entertainment Book, then I would say that you could get away with not buying the DC.
You can get similar savings on admissions if you are a AAA member or at several online sites (except on APs - if you want APs - you should buy the DC - it is the only way to get a discount on them.)
You usually can find discounts on car rentals elsewhere.
I bought DC last year and still ask myself if it was worth it. But when you add in the $15 that I saved on my AP and the savings that I got shopping, then I have gotten back my $30 and more.
I hope that this has given you some food for thought...