That's not too far removed from relying on WebMD to cure your cancer.
What? It's a million miles removed. First of all we can't even cure cancer, so it would be like doing something in law that is impossible for anybody to do. But let's assume you just mean something very difficult to do, like operating on a malignant tumour to get it removed. Even then, this would would be more akin to, say, representing yourself in court in a $10m case.
But WebMD isn't even set up to help you operate on a tumour; it's there for self-diagnosing things like sprains and tummy aches, and giving you an idea for when you need to go and see an actual doctor and when it can be treated at home (or ignored completely until it goes away). It's also there for friends and relatives of patients suffering with more serious illnesses to gain a better understanding of those illnesses, the patient's needs, and what they can do to help out. It's specifically designed for laymen to use to help them understand the types of medical problems that directly affect them, and for them to act on them in the most appropriate manner. Indeed, these sorts of sites are so effective that the National Health Service in the UK set up
its own symptom checker and medical database, to help people with things like stomach aches decide what course of action to take - to see a doctor, take this pill or that, or just drink plenty of fluids.
Using a will writing tool is clearly somewhere in between these two things. But if your estate and wishes are as simple as a tummy ache, then buying a kit from WH Smiths for £10 is perfectly fine. For a small number of people, e.g. people who want to leave their whole estate to their spouse or, in the event of their spouse's prior death, to their children, such a will is akin to a tummy ache, and can be written using standard templates very cheaply.
Clearly, for a lot of people who might have a larger estate or want a more complicated will, using off-the-shelf templates is a bad idea. And this is where it differs from WebMD in ways that are valid to your comparison. If you look at the probabilities, I would say that the following is true:
1) The probability of having a minor medical problem is much, much higher than having a major medical problem
2) Even though the probability of complaining about a problem is higher with major problems than minor problems, the probability of "Minor problem" given "complaint" is still very high (much more so when the complaint is made
to me in place of an actual doctor - if someone's moaning about it to me instead of seeing a doctor, then it's probably not that bad)
3) Even if the problem is major, the NHS website/WebMD will correctly recommend visiting a doctor in the appropriate circumstances
Therefore, I would recommend using NHS website/WebMD, rather than booking an appointment with a GP, in most situations where I am likely to make such a recommendation.
However, the same isn't true of will writing. The probability of having a major-will writing requirement is much higher for most people. And if you're looking to write a will in the first place, then the probability that you have a major-will writing requirement is higher still. Finally, a will writing pack won't tell you if you need to go to a solicitor: it is simply a template that you fill in, not a comprehensive database of will-writing scenarios and advice. Hence, I would agree that using a £10 will writing pack is probably a bad idea for most people.
But your comparison is really not a good one, first because it does WebMD a disservice, and second because a £10 will writing pack might well be perfectly adequate for some people. And at the very least, the probability that your needs are no more complicated than a £10 will writing pack is capable of handling, given that you have written a will using a £10 will writing pack, is probably quite high.