I listened to Thanissaro Bhikkhu's talk entitled "the problem with Buddha nature" last night. He likens the concept of Buddha nature to a new tree in your garden that kills of some of your old flowers and presents two arguments to support this assertion: 1. That the concept of Buddha nature is superfluous, that is has been introduced in Mahayana 800 years after the Buddha, and that people can become enlightened without the idea of Buddha nature. 2. That the notion of Buddha nature might lead to misunderstandings which arrest the practitioner in certain (detrimental) states of spiritual complacence. He then proceeds to outline three ways how this might happen.
While the talk is very enjoyable and erudite, and is recommendable for various reasons, I can't find myself agreeing with either of these two arguments. The first argument can be applied to pretty much any Mahayana tenet, as it essentially boils down to: "It's not in my book." Although none of Thanissaro Bhikkhu's statements are false, they do neither invalidate Buddha-dhātu. I mean, people do get to enlightened in both the Theravada and Mahayana traditions, as well as in Hinduism, Jainism, Christianity, Islam, etc. - you get the point. The fact that something is "not in my book" does not invalidate it.
The second argument has a little more weight, and it was actually mentioned first in the talk, as it points out the various misunderstandings that can result from the notion of Buddha-dhātu. For example, people might get the idea that they are already enlightened, or that enlightenment is an automatic unfolding process comparable to evolution, or that progress on the path is due to grace rather than effort. Certainly we have to agree that this might mislead people, but what the heck? The same can be said for almost any concept in Buddhism from the four noble truths, the three marks to the khandhas, the pāramīs and what not. Traps and pitfalls are everywhere.
The value that I see in the idea of Buddha nature is that it provides impetus necessary to go all the way. For a Buddhist practitioner, after having dropped the gross attachments and after having developed some skill in meditation, it is fairly easy to become comfortable in samsara. With some practice one can achieve a level of comfort and bliss that is comparable to that of the devas. Obviously, it would be a mistake to stop there. The awareness of Buddha nature provides the necessary inspiration to go beyond these states and as such it has a soteriological significance; unfortunately, this point was not addressed by Thanissaro Bhikkhu. Neither did he address possible ontological interpretations of Buddha-dhātu, which are equivalent to ātman and generally rejected by Buddhists.