Is an MFA -as in a Masters of Fine Art needed to be professional artist? the short answer is no. Does it help? yes. So I guess the real questions is: how does an MFA help in an artists career? I will start by saying I dont possess one myself so I am not speaking from experience, so stop reading if you want first hand info. I have however visited numerous MFA grad shows and colleges, in London, US and Canada. I sat in on a class in NYC, been to grad shows in NY, Rhode Island, Michigan (Cranbrook), Goldsmiths, the Slade, the RCA and St Martins in London and numerous schools in Canada. So I am not uninformed in terms of the quality of art produced.
I recall speaking to a graduating student in Montreal's Concordia University which had a good reputation at the time (in Canada). I recall her telling me she was going on to study computer animation as she didn't really see the point of the MFA, so just as this one particular example, there are another few 100 students who graduate from MFAs every year and may or not even pursue an artistic career.
From what I can gather here in UK especially in London an MFA is invaluable in terms of getting exposure. For one you get to participate in shows, the graduate one being the most obvious but also others while in school. You will also get gallery owners coming around to the grad shows looking for potential up and coming artists - but of course its not like that can take everyone on.
I also think the MFA affords you a time to think about your art -and do only that - as well as getting others in the same situation to do the same. I do think you need to be in the right frame of mind however as not being 'into it' while doing an MFA will mean important opportunity lost. So as an opportunity to connect and network with potential galleries and like minded people I think an MFA is invaluable. My only beef with an MFA is that sometime they are looked at as 'credentials' for making art.
You don't need any obviously, but in the current Art climate where it is sometimes difficult to tell the difference between Art and non-art, people like anything that gives them a little bit of assurance that the artist is serious about his or her work and I think and MFA gives people that impression. Like say, someone who has a graduate degree in botany would be more serious about plants than your average horticulturalist or at least that is the impression you would get without meeting them.
So in an nutshell - in terms of an art 'career' if there is such a thing - a MFA will be of help - but not in terms necessarily of artistic development and I would even argue that it could sometimes be a hindrance to artistic practise when it's influence stops you from working.. ie; gives you so much to think about you become overwhelmed or paralysed by the possiblites or questions posed or even takes you in a direction that is not naturally your own and it ends up being fruitless.