If you dont have a credit card, it does the job if you want points
However, Microsoft does it (for Canada at least) so that when you redeem points on Xbox LIVE, you will always have a little bit of remnants in your card that can't be spent in order for you to buy another one some other day.
Example: You buy a 1400 point card.
You buy Splosion man, or any other 800 point game
You then buy Chime, or any other 400 point game
Great! Now you have 200 points which you cannot spend!
Some might say you can spend the points on Avatar accessories, but 1) who the hell wants to pay for these things 2) the prices are in increments of 80 points, so you'd still be left with 40 points.
The problem with this is the fact that most games and downloadable content are priced in increments of 400/800/1200/1600 and so forth, while Canadian cards are 1400/2800.
I wish we would also have 1600/4000 like Americans. Plus, we pay 20$ for 1400 points while they pay approximately the same for 1600 points.
Or, even better, it would be great if XBL switched to real pricing just like the PlayStation Network.