Doulas are trained to attend to YOUR needs/wants. Doctors, nurses, and midwives, while they can be very good at supporting/encouraging you, and have your best interest (healthy mother, healthy baby) in mind, they can inadvertently leave you feeling emotionally neglected . If something unexpected happens, believe me, EVERYONE (including hubby) will be more focused on how you are doing below the waist (the focus will be almost exclusively on the baby), and give you short shrift above the waist. You may be nervous, scared, tired, frustrated, or worried. The doula's main focus is YOU. If you need reassurance, or comforting, or rallying, or help communicating--she's there for you. She will sneak you ice chips should you end up with Nurse Ratched, give you a foot rub, hold your hand, adjust your pillows, and basically tend to you. Your husband may be able to do some of these things for you, too, but oftentimes fathers (esp new ones) feel just as nervous and helpless/lost not to mention exhausted (from dutifully staying up all night with you) as the laboring mom. And if he doesn't have a background in medicine, being surrounded by all the technology and uniformed technicians--esp in the case of a difficult labor or an unexpected complication--he can easily be overwhelmed, and you may see the once strong man you know reduced to a scared little boy, only too willing to cede all control to the experts in the room. That is when you will need someone who has the experience and knowledge to help you get thru it all, someone who will not feel like a fish out of water, someone who has been the situation before and knows what's around the next corner, someone who anticipates what your needs will be.
Can you get by without a doula? Sure, but personally I would make that call with baby #2. Go all out with baby #1. It is with the first that you usually figure out what all you need/want for the next time around anyway. It's better than woulda/coulda/shoulda when it's all done and over.
edited to add: and should you decide to birth at home, the doula shows up first, helps you thru early labor, and will know when it's time to call the midwife.
Also, scipio was not at all happy about laying out so much $$$ on a doula, but he was more than convinced afterward. It took a great deal of pressure off of him.