Why list your phone number in adwords?

I posted this in the main Google optimization forum at SEO Chat, with no response, so I'm posting this here, hoping some more interested people will see it and respond...

To anyone who feels inclined to answer:

I was doing some research for my company yesterday morning, and noted that some firms advertising on Google have opted to place a phone number in the ad text.

For example, in a search for "riverside ca plumbing", I noticed the following ad:


Need a Low Cost Plumber?
Midtown Rooter Plumbing. Call Now
818-555-5555 - 24 Hour ER Service
----URL address-----
Los Angeles, CA

There were several others like this in other searches that I tried, and most of them seemed to be from companies that are one-man-shows or at least pretty small.

My questions:

1. What are the benefits/drawbacks of putting a real phone number in the actual ad text?
2. Why doesn't Google block this? Or, can they even?

It seems to me (a) that, from a web design perspective, this would defeat the purpose of advertising on Google, since people might be "converted" without actually clicking on the link, and so your click-thru reports would not actually show the real success of the ad. I would think that any web designer would never practice something like this, if they valued the future of their career.

Also, (b) it seems that this is a way for the business owner to get around paying for all the customers he gets through the ad, and therefore Google ought to block this practice if they can.

Any thoughts?