Kenneth talks in the comments about working with the spirit of the entheogenic plant or fungus as a part of the ritual. This is absolutely one good reason why someone might work with entheogens. It is, in fact, one of the reasons that I do so as well. Touching the living spirit of such a powerful being is an profound experience when it works. And yes, once you've got to that state, you may or may not ever need to actually use the entheogen again to re-experience it or to have a good, solid contact with that spirit.
This, though, was immaterial to the point I was trying to make in that post. My point there was that failure of ritual can help us learn a lot of different and useful lessons. Failing means retooling the work and trying again, or repeating the experiment to see if something about the set and setting were problematic. Failing means reevaluating where you are and where you're headed. Failing means dealing with disappointment and losing the sense of being a special snowflake whose every action is fraught with spiritual significance. Failing means recognizing we're human and that we make mistakes, or that even if we haven't made a mistake, sometimes things just don't work the way we expect them to. Failure offers us chances to grow and mature.
What really rather gripes me, though, is anonymous's assumption that I've never read either anything by Stewart or the Matthews' (I have a great deal of their stuff on my foo-Celtic shelf) or anything from the original Gaelic and Welsh source material. I've read a lot of sources on Gaelic pre-Christian religion and culture. I've read a lot of the medieval manuscripts in translation. I've even done translations of materials from Old Irish myself, for my own understanding. Hell, I've had my translation of the Cauldron of Poesy published at least twice. A lot of my work has been translated into other languages.
Much of what the Matthews' reprint is 19th century scholarship, outdated in the mid- to late-20th century. While there are occasional useful nuggets in their reprints, I've already read the vast majority of what they're offering. And I disagree with a lot of their interpretations and uses of the material. I don't find RJ Stewart's ceremonial magic approach to the materials very useful for my own work either. While the Matthews' do a somewhat better job of dealing with Celtic spiritual material than, say, DJ Conway, Edain McCoy, or Douglas Monroe, it doesn't mean they are presenting the source material without their own particular filter -- in their case, usually, Celtic "shamanism", which is a rant in and of itself.
Certainly neither the Matthews' nor Stewart talk about deity in any polytheistic fashion. The Matthews', in their Western Mystery Tradition books, refer to the Celtic deities as "unregenerate godforms," whatever the hell that's supposed to mean, and warn against working with them. Perhaps this is their way of saying the deities are dangerous. If that's the case, so is fire. So is the sea. So is walking out your door every morning to go to work. Should we stop heating our houses, cooking our food, and going outside because it might be dangerous? People are dangerous, too. Even our closest friends and the people we love might hurt us from time to time. Do we stop having friends and family because of this, or do we learn to deal with their rough edges and accept them for who they are?
Ultimately, I write about entheogens because I find them useful in some ritual circumstances. I write about them to show that there are many ways to define and practice rituals. I discuss my experiences, both successful and failed, in order to demonstrate that effects vary and that not everything is going to work every time. In modern US culture it's hard to find people openly discussing that kind of work. Discussions go on in closed fora and between friends in private. There are books out there but the good ones can be hard to find. And to neglect such an ancient source of access to ritual states of consciousness and such powerful potential spiritual allies is to cut oneself off from the potential for powerful learning and spiritual experiences.
Yes, there are dangers inherent in the path. People have averse reactions to entheogens just as they do to any other substance they may put in their bodies. For some people, being in the vicinity of onions is a life-threatening experience. Strawberries have killed people.
Important things to remember are to research thoroughly, to exercise due caution, and to understand that these things will not always work as advertised. Sure, you don't "need" to use entheogens in a spiritual practice. One doesn't "need" a lot of things that are or can be useful. It doesn't mean they should never be used by responsible adults. And it doesn't mean that non-entheogenic practices can't get you where you want to go as well, depending on your goals. Humans are curious creatures. We are interested in new experiences and are prone to experimentation in all fields of life. Ritual is like sex, in the end analysis. Different things work for different people and my kink may not be your cuppa. The important thing is that we all try to get to our goal, by whatever pathway pleases us best.