You want your business card to be the best little rectangle it can be.

A thin, flimsy card is like a dead-fish handshake. You want it to have a good feel to it, clean edges, rich color and clear images. Plus, you want it to convey to your customers that details matter to you.
You may be tempted to use an online program (like Vistaprint) to create your own cards. After all, you know what you like and what you want your cards to say. Using these programs, you can probably make decent cards and print them for a reasonable price.
Some people, in certain situations, can and should go ahead and use the online programs. But not most. The main reasons you should have a professional assist you are: design quality, design consistency, originality, brand integrity and integration, and finally, printing quality and control.
Design Quality
Some designs are superior to others. Your industry/profession may impact which designs are more effective as well. While your personal preferences and expression of your unique business matter, a professional designer has more experience with identifying good designs and less good designs. Designers know how people will respond to visual stimuli and can help you select a high-quality design, rather than one that is “good-enough.”
Design Consistency
Designers are detail oriented in ways that most people aren’t consciously. You may notice an ad and feel like it’s “off” or “not quite right” and not know why. A designer usually can tell at a glance when something isn’t lined up, the size is wrong, or the proportions are off, as well as when items within a brand aren’t consistent. When you have multiple documents and materials, it matters that they all use the same design. This is not to say that they all have to be identical, rather the designs must be complimentary. Consistency and complementary design is tricky to obtain on your own.
Originality
The odds are high that if you use an online program you are going to use an existing template to design your card. It’s easier and it already looks pretty good. Usually you can pick from a number of colors or make small changes, but the card won’t be unique to you or your business. Given the fact that these designs are low-cost and readily available, it’s entirely possible (likely even) that you will someday find yourself exchanging cards and noticing your cards look nearly identical. That may be a funny bonding experience, but what about when you and another person each give the nearly identical card to clients? Clients may not remember who is who or, more likely, go with another company who offers a more memorable, notable presentation of themselves.
Brand Integrity and Integration
Your branding matters. Whether it’s a new business or an existing business, everything should be cohesive. Repetition and consistency help reinforce your company, your professionalism, and your product or service. By using different sources for different materials for your company, you risk inconsistent use of colors, logos, fonts, designs, and materials. When those materials come together and don’t match, they clash and look shoddy.
Printing Quality and Control
You get what you pay for with online printing. It’s not necessarily bad quality, but it’s also not usually great. Cards might get cropped (cut) a little bit off, or colors that don’t print true. The colors probably won’t be as expected, since what users see on a screen won’t match what a color looks like on paper. A card is your lasting impression you leave with a customer.
Your business card is one of your most important printed materials. Don’t settle for a mediocre product when you can have a professional quality card for not much more money. The investment you make in your presentation is quickly paid off when you gain even just that one extra client.