It seems easy to argue that personal therapy has value for several rather banal reasons: the experience of being in the “hot seat” (being encouraged to divulge personal details, opening up about your insecurities) can give you a sense of what you are asking people to do. The opportunity to discuss feelings about the ways your work affects you personally is also a way to ward off professional fatigue and "burn out." The privacy of the whole situation (it's hard to see what a therapy session is like without actually sitting in one) also makes personal therapy seem an attractive introduction to how the process ought to look - a sort of apprenticeship.
There are also arguments that the case in favour of personal therapy is overdone. It is a very resource intensive requirement, hitching entry to the profession to a large financial outlay that some more "hard headed" clinicians argue is simply unnecessary. I sought therapy when I was training to be a therapist, but whatever value it had for me as a person, it was not the only (and maybe even not the best) source of insight about the intersection of my personal and professional development. Intensive supervision on the other hand was profoundly helpful. I had the fortune to meet with supervisors who took a granular interest in the blow to blow of the sessions, and who were not afraid to tell me when they thought I was avoiding material from my own anxieties, or getting wrapped up in a response that was more aimed at gratifying me than helping the person I was meeting.
Looking at my colleagues I hesitate to suggest personal therapy is essential. Some of them were the most gifted and emotionally insightful people I have met, but I suspect this was principally a result of their temperament, interests and life experiences. Many psychologists seem conscientious and rather neurotic, making them good candidates for extensive self-examination in or out of therapy. Others I have met seem somewhat emotionally unaware despite years of personal therapy. Hardly a convincing advert.
It may be that this is a case (one of many in my view) in which the psychoanalysts have made an accurate diagnosis of a problem (therapists without personal insight are a bad thing!), but don’t have a monopoly on the remedy. Much as “mindfulness” is actually a description of a wide ranging mental state, which can be facilitated in many ways other than those that have been made popular since the advent of professional meditation training, so "emotional insight" does not only need to arise from a particular form of two way conversation that became popular in the 20th Century. The flip side of this is that therapy is not a precise science and has no guaranteed outcomes anyway. It would seem weird therefore to put too much store by it as some sort of royal road to clinical wisdom.
How could we settle the question? Apart from examining the efficacy of therapist therapy on outcomes (those who worry about the value of personal therapy for trainees often ask to see the evidence that it "works"), the most interesting way of looking at the problem would be a straightforward “taste test”. You could (for instance) take a panel of senior psychotherapy teachers who run therapy courses mandating personal therapy and have them interview a sample of psychotherapy trainees, some of whom have had at least a year of therapy and some of whom have had none. Would the trainers be able to discern the “analysed” from the “unanalysed” cases? If the trainers’ judgments about who had had therapy were no better than chance then that would seem a significant challenge to the dogma that personal therapy is doing something clinically relevant.
Ultimately the dispute is so intractable because what is at stake is two conflicting visions of the ways that it is possible for us to deceive ourselves. For the cognitively minded, a salient sort of self deception might arise from the various self-justifying biases that are tied up with the insistence upon personal therapy: that it may seduce you into thinking that you’ve got (that it is possible to get) your own psychological house in order; that it bestows an unchallengable authority on the figure of the "well analysed" therapist ("I have the requisite sort of insight - you do not"), and that it is an arrangement principally suited to keeping psychotherapists in business through a steady flow of trainees who need to sit on the couch.
For the psychoanalytically inclined, the relevant self deception is tied up in the hubris of embarking on "the work" without sufficient knowledge of what lurks in your own unconscious. Enter the room as a psychoanalytic naïf and you will be hit by a storm of transferential and counter-transferential responses that you can’t make sense of. You will likely be pulled into a range of potentially harmful enactments with the people you’re trying to help. You'll be lucky to avoid causing harm to the client and to yourself, let alone providing help. I find this way of thinking fairly compelling. The rampant abuse of patients by mental-healthcare professionals (who didn’t – I assume – enter their field as aspirant sadists) looks like striking evidence of how harmful and surprising our unconscious motivations can be. Personal therapy - under the psychodynamic conception - is a way to start looking at the darkness that lies within you so you can stop it from wreaking havoc on those you work with.
The point of raising these two types of consideration is not to try and arbitrate between them, but to diagnose the whole problem with the debate as it is currently constituted. Both sorts of concern seem valid to me. I am convinced of the fact that therapists bring things into therapy that will impact on the work in potentially profound ways. I am also convinced that therapists are motivated to preserve their professional identity by appeal to the special significance of their hard-won clinical insight, and that this can often be overblown. If different sides can retreat from their favoured assumptions, a new way of asking the question might emerge.