The biggest problem when using these types of secondary marketing techniques is that people don't have a firm idea on how to use them, so they tend to just use the technique for the sake of having it.
In regards to a web presenter, just having one talking about nothing can be very annoying and completely undoes what it is you are trying to do in the first place, as you have pretty much pointed out from your experience.
This type of marketing as to be pointed, just like any other form of good marketing. If it's not pointed, then it's usually dollars down the drain with marketing.
Just to quickly answer your question, I am not currently using a web presenter, however it is an option I have been considering and one that I am going to go ahead with in regards to a couple of ventures I'm involved in.
For me, the best way to use a marketing tool like this is one of two ways. A short, but warm welcome to your site, that involves inviting the customer to contact you if they require further assistance.
Secondly, to highlight certain aspects about what it is you offer and why they should purchase from you.
Those are the two areas where I see the value in a tool such as a web presenter. Having said that, I think it is important for the customer to have control over the presenter, in so much as having to option to turn it off if required, especially if your site is all about getting repeat customer visits.
These types of tools can get very old, very quickly, when a customer is visiting more than once, so always bare that in mind.
Really, you pretty much have a bank canvass with a web presenter. You can choose their dress style, the way they enunciate phrasing and so on. So if anyone is going to use this type of tool, make sure you fine tune it to meet your target demographic to get maximum bang for your buck!