Over the years I've met a variety of people who claim that one needs to eat a vegetarian (or vegan) diet to continue their spiritual evolution. While it certainly sounds 'nice' that one is moraly and ethically advanced by not consuming another 'intelligent' life form, I find it difficult to agree.
The concept that humans are the most advanced beings who have a moral, ethical and spiritual obligation to 'protect' the animals doesn't stack up if you actually practice magic and experience non-ordinary states of being. For example, as many indigenous cultures discovered, plants too are powerful teachers (allies). The fact that plants do not move around, show emotions, or communicate on a physical level relegates them to a lower rung on the hierarchical ladder (from a generalised Western perspective).
I am not suggesting that the current treatment of animals as a commodity (large scale farming) or as entertainment for humans (zoos, pets) is acceptable or spirituality enlightened, for it certainly isn't, and a lot needs to change. What I am suggesting is that sentience isn't solely confined to the physical plane, so to make a judgement as to the state of advancement of a being solely by its physical attributes is flawed, and mainly due to the intellectualisation of spirituality, rather than through the experience of spirituality.
I have found it relatively easy to work with plants using trancework, my main work being with cactii. It isn't even necessary to consume the plant to be able to communicate with the powerful spirit behind it. Through this experience I would find it difficult to justify eating plants as a 'superior' means of sustenance over meat, nor is it necessarily 'less violent' (as some people claim).
For me, it is the awareness and intention behind our actions. If we eat out of necessity, with respect that something else must die for us to survive, then we are already living a 'more spiritual' existence, as any increase in our awareness is part of our spiritual journey. As our body is an important part of our total being (the physical aspect of our Self - 'thick spirit' some say) our diet should be based around considerations of our health, and if that dictates primarily vegetable and plant material, then that is okay, as is more meat, when that is necessary for our well-being.
The arguments are long and endless as to what is 'right' and 'wrong' but ultimately it is through personal experience that we can transcend this 'right' and 'wrong' and find what we need. Life (and hence magic) is not black and white.