Recently, Grimmly posed the question: Is an assist a prop? As always, Grimmly's interesting question has generated a lively conversation on his blog. Claudia thoughtfully commented that an assist is a prop, but at the same time "is more than that".
I think this is right. I would go further, and say that an assist is, in a sense, superior to a prop. Why? Used properly, a prop serves two functions: (1) It enables you to get into a certain modification of a particular pose that you might not be able to get into while maintaining proper alignment without the prop, (2) It enables you to feel and "get" the required opening that one needs to get to perform the posture effectively.
Let's take Kapotasana as an example. There is a tendency for many practitioners (including me) to bend too much at the lumbar spine and/or the shoulders in this posture, while not taking enough of the backbend in the thoracic spine. The effective use of a prop (say, doing a preparatory backbend with a block wedged between the shoulder blades) can function to remind the practitioner to take the backbend more into the thoracic spine.
However, in my opinion, the trouble with props is that over time and with repeated use, there is a tendency for the practitioner to come to rely too much on the prop to open the area/s of the body that needs to be opened; when this happens, the practitioner consciously or unconsciously comes to see the prop as a purely mechanistic device to be used in a mechanistic way to passively open a certain part of the body in order to "get" to a particular posture. When this happens, the energetic flow of the asana is overlooked. The same thing tends to happen when one uses blocks for forward folds or to rest the lower hand in Trikonasana: There is often a tendency to lean into the block, and in this way, neglect the energetic engagement of the bandhas that is integral to the effective performance of the posture.
Seen in this light, an assist is "superior" to a prop simply because the assistant is alive whereas a prop is "dead". A properly trained and sufficiently perceptive assistant understands that the purpose of the assist is to enable the student to feel the energetic flow of the asana while developing the strength and engagement necessary to eventually perform the posture unassisted. With this understanding, the assister can provide feedback to the practitioner consistently, reminding the practitioner to keep working on cultivating strength and engagement.
That said, I think it's possible for assistants to over-assist. For instance, sometimes people can so dependent on being assisted into a particular posture that they may not develop the strength to get into the posture themselves. Because of this, there is good reason to give assists sparingly: For instance, at her Richmond workshop in April, Kino told me that it is better to let people struggle with Utthita Hasta Padangusthasana at least a couple of times rather than just rush over and support them from the beginning. Same goes with Kapotasana: If one never had the opportunity to try and get into the posture on one's own, regardless of how deeply one gets, one might never develop the strength and muscular and energetic awareness needed to perform the posture effectively.
This, at any rate, are my two cents/rupees/yen on the topic.