I love it when people make arguments that a particular technique is the one true way that magic works. Or make the argument that the only way something works is through the intercession of spirits. Let me be clear, I do evocations, invocations, and work with spirits, but not all the magic I do involves spirits and I don't always get instructions on what to do or how to do it from spirits. I feel that evocation is much like any other technique of magic. There's a time and place for it and knowing when to use the technique is as much a part of what makes it effective as anything else.
Jason Miller shares his own perspective by arguing that we are spirits too. Much like him, I've never just practiced a Western Esoteric approach to magic. And Eastern practices emphasize the cultivation of one's own spiritual resources and power. But even without that influence, I'd argue that Western magic doesn't entirely support a spirits only approach to magic. A read through William G Gray's Magical Ritual Methods or Franz Bardon's Initiation into Hermetics shows a similar cultivation of one's own spiritual resources, both in relationship to connecting with spirits and in relationship to doing spiritual work that doesn't involve spirits.
I respect the spirits I work with and I'll acknowledge how they have impacted my life at any time. But I also have to acknowledge that I have done a lot of the work and that the work hasn't always involved the intercession of spirits. When its necessary I do work with spirits and magical entities to help me accomplish certain tasks or approach a situation from a perspective that isn't my own. But sometimes its also necessary to do magic drawing on your own experiences and resources. The wise magician recognizes that spirits, inner contacts, entities, etc, are allies who nonetheless expect you to do a lot of the work on your own. S/he also knows that its important to cultivate his/her own power and experiences and apply those to magic without always looking to allies to fix situations.
And to be honest, not all my evocations have involved spirits. Sometimes I've evoked people into my life, or evoked situations. Indeed, I think that evocation as a technique is quite useful, but much like any other technique it can be experimented with and improved upon if a person is willing to test it and see just what s/he can do with it.
There is no one true way to do magic, no one true technique that trumps everything else. And while spirits are powerful and should be respected, so are we. We are spirits manifest as flesh and we have something the spirits don't have. The ability to interact with the world in a much more direct way than the spirits can. In truth, the spirit human relationship is a symbiotic relationship, with both sides relying on each other. When we recognize this, we recognize as well as that what makes magic effective is as much the person as the spirit.