I use the IDEA Newsletter, and it is a marketing tool of sorts. I use Constant Contact as well for client contacts. The address list is pretty much like the one for IDEA but, of course, I am the creator of the content, and, as such, I consider this as my marketing tool.
I do not use a newsletter as a marketing tool, but I do send one to my clients (I use the IDEA template). They seem to really enjoy receiving it so far, so perhaps it could be viewed more as a bonus and special addition for them–and maybe even retention tool if geared specifically toward them.
I would also be curious to see if others use a newsletter for marketing.