A good doula - one who knows the ropes, the system and how to NOT intrude on the bond between your and your partner - is worth her weight in gold, and then some.
You might get lucky and have a relatively easy/short labor card, cooperative nurses, a doc that you mesh well with and hospital policies that are in line with your beliefs about birth. And you might not. If you are in labor for 36, 48 OR MORE hours, how is your husband supposed to support you that whole time, by himself? And who is going to support him? To remind him to eat and drink and give him a chance to walk away and go potty, get a breath of fresh air, etc.
Most dads want to do right by you, but when they see the love of their life hurting, all they want to do is make it stop and they usually don't know how. Maybe you'll be the mama that will keep it all together, labor quietly without too much discomfort, and maybe you wont. Maybe you'll be LOUD, scream and moan and ask him to make it go away even though what you really need is some reassurance that everything is working perfectly and it will be ok.
There are a lot of doulas who don't know how to do their job with grace, but the ones who do are absolutely invaluable. If you're delivering at a hospital you have no idea what your nursing staff is going to be like until you're there - you might get an awesome nurse you supports your birth plan, is as hands on or off as you need her to be and isn't pushy. Or, you might get the nurse who thinks it's ridiculous for any women to go through labor without pain meds (no one gets a tooth pulled without meds, right?) and therefore feels the need to offer an epidural or "something to take the edge off" every time she walks in the room. Will your husband know how to handle *that* nurse? Probably not. But a good doula will! Will your husband know what positions you should try laboring in? Or remember to recommend a potty break or sip of water regularly?
A good doula will NOT interfere with your husbands ability to help you - she will mostly likely give your partner suggestions on how to help you more effectively. Many doulas (in my area at least) will come to your home in early labor before you're ready to head to the hospital and help you determine when might be the best time to go in. (Getting to the hospital too early can lead to unnecessary interventions.) Many are also *trained* in added services like photography, massage therapy, aromatherapy, TENS units, Rebozo, lactation aid, birth classes/philosophies such as Bradly, Birthing From Within, Lamaze, Hypnobirthing, etc or have additional training for special circumstances (adoption, c/s, single parenting, LGB parenting, etc)
There is NO CHARGE to interview a doula, or 10 doulas! I recommend you interview SEVERAL and see who seems to be a good fit for what you're looking for. Not all "seasoned" doulas are good doulas, and not all students are bad ones - interview a bunch and go from there. They should be able to tell you what makes them special/different and WHY you should work with THEM. They should also be able to provide references to other clients so you can call and ask them what sort of impact they made during their birth.
Here's a neat story to read: http://birthwithoutfearblog.com/2013/01/05/the-difference-a-doula-makes-a-birth-story/comment-page-1/#comment-12517