When you find that special person, commit!
Sounds like good relationship advice, but it also applies to business. Do you like your dentist? Your hair stylist? If yes, chances are you keep going back to them for more than one reason: you trust them, you feel that they care and listen to you, and they know your dental (hair) history. Think of your branding designer the same way you think of your dentist or hair stylist.
Having one person work on all your marketing materials ensures visual continuity. Your designer is familiar with your branding strategy and will base any new projects on the files he or she has previously created for you. That means, you will get the same colors, the same fonts, the same overall look, and that is extremely important for good branding. And if you need a file, you know who has it.
If you divide your projects between several designers, let’s say you use Fiverr for a logo, a print shop to design your business card, and yet another designer for an ad, then nobody is really invested. It’s just a job they get paid for and then they move on. Consistency in branding is probably not their priority.
Plus, for you as an entrepreneur or business owner, time is money. You don’t want to explain every time you do a new project with a different person what the look and feel of your brand is. You need someone who is on it, who tells you what’s the best way to do it. Best example again, your dentist. He will say “We’ll need to fix this tooth and this is how I’m going to fix it for you.” As a business owner you have so many things to do. Running after files or having to micromanage an unqualified person should not be on your list. Delegate your visual branding to an experienced, trusted designer.
And I'll tell you a secret: good designers are always updating their portfolio. They'll want to make your project one of their show pieces. Their reputation depends on how well they design for you. My motto is "Your Success Is My Success." We're in this together :)