Ian's words ring very true: we are not shepherds, we have no flocks.
The "ritual specialist" language is too simplistic, though, and speaks to a very limited role. I'm not overly pleased with it, so I expect to mostly abandon it soon, or at least modify it with additional terms.
Over and over, though, I come back around to three things: partnership with laity, commitment and training.
The commitment is really the central piece; training is an addendum, really, that leads to obligation and commitment, a sort of *ghosti relationship where when you do the work to get trained, you find yourself committed to training others in the ways of Our Druidry, and to use the skills provided by that training to ensure that the proper rites and sacrifices are made in the proper way at the proper time. In many ways, an ADF Priest is first and foremost a person committed to serving the folk through their training and making sacrifice to the Kindreds.
In any case, there is a two-way commitment: one to the Kindreds, and one to the Folk. I am generally not of a mind that a Solitary member cannot become a Priest, but I think about the oath I took and I wonder how I could fulfill it from a Solitary point of view. I also, though, accept that every individual will find a different vocation, and that they will be called to serve the Folk in different ways. Because of that, my own notion of how someone else might fulfill their oath is fundamentally flawed: not knowing their vocation, I cannot question their fulfillment of it effectively.
It occurs to me that there is also an ineffible quality to priesthood, within ADF and (I suspect) outside of it: those who have been through the processes of consecration and ordination (and now, initiation) will view their role differently than those who have not passed those boundaries and have not made those commitments, and differently than those who have not done the training. I suspect that there is an element of intent to the training, as well, since a trained Liturgist might find similar courses but a very different experience in them, as I know I have.
I think that, because of this, I can only see the role of the Priest in ADF as a partnership of sorts: just as you cannot define a "partner" in ballroom dancing without discussing dancing, one cannot define Priests without discussing the relationship and interaction between Priest and laity. If we consider separating the Priest from the laity at all (either by defining one group as "useful" or one group as "useless"), each category must understand and express the value of the other in order to find value within itself.
A book that I love describes it like this: "We can ask, What is a wombat or an edible dormouse?, but not, What is a priest? The priest is no independent species – the 'laity' are part of the picture of what the priest is, and the priest is part of the picture of what the laity are." The notion that priests can be defined in isolation from the congregation is a ridiculous one, and I suspect that the true issues that surround "defining a Priest in ADF" likely revolve around insecurity about what an individual's role is in relation to the ADF Priest, not insecurity about the role of the external priest.
My initiation into the ADF Initiatory Current was a very reflective thing: I am deeply engaged in retrospective thinking and considering new things that are coming up. My actual essay on this topic (meant for Oak Leaves) is already stretching several pages, because the question of the role of ADF Priests is so very complicated, so I won't dwell on it here much more.