Thats a great question, whether just purely working the mental aspect of evocation can be as effective as working both the mental and physical aspects of it.
When you come down to it, its all about the mental/emotional state of the magician when performing any kind of ritual, whether it be evocation or otherwise. Without the mental/emotional state, all the physical actions and words spoken are empty and meaningless. The words themselves dont have power, nor do the movements or tools, in and of themselves that is. They are foci for the magician to concentrate upon and get themselves into the proper state of mind (mental/emotional in this case) so they can more naturally open their psychic faculties for communication with entities that have no physical basis, for all intents and purposes.
IMO it is a great achievement to be able to communicate psychically without having to coax yourself into the state of being open to receiving and transmitting. Some people are naturals at this; most of the rest of us need to work diligently on it for a long time to get anywhere.
So to whether its as effective or not, it depends on how well you are at getting yourself into a focused psychically open state without having a physical stimulus to aid you. If you are good at it, and in a meditative state you feel open, receptive, clear headed, and can feel the presence of the spirits just as if you did a physical ritual, I would say it is self-evident that it (whatever technique you are using) is just as effective as a normal ritual.
We should remember always that the tools we use are symbolic. Indeed the entire physical plane is a symbolic emanation from the spirit plane (to put it simply). So while a tool is useful for focusing the mind of the magician, the tool is just a symbol of the essence beneath. If one can directly tap into the essence of the tool, the symbolic representation of the tool is no longer strictly necessary.
All magick is fundamentally psychism. So if you can achieve the proper psychic state through say just meditation, that you can with physical action, it is only your own doubt and distrust in yourself that could cause discrepancies between results.
All IMO, of course.