Mashups are cool. Cool to develop. No denial. But what about the person accessing your mashup? Is their browser being hung up? Their bandwidth pounded with log of requests (be it AJAX or otherwise)?
A good mashup should pay attention to the usability aspect as well. Just pulling in information from lots of websites and assembling it on the fly on the client-side may not work. Especially if the information being fetched from the remote site has to be repeated for each row on a table of data, a standard implementation would definitely choke the browser.
Take Online Ad Networks page for example. It currently has mashups to get the Google’s PageRank and Alexa’s traffic rank. However, if this information is loaded on the fly for each row in the table, not only is it going to be painful for the viewer, it also would rapidly fire several requests to the respective servers from where the data is being fetched. So, instead, the information is loaded on demand, through the onmouseover events.
In general, in cases where a lot of requests have to be sent, instead of doing that by default, it would be good to do it on a user action. The choice is typically between a onclick and onmouseover. If you want to avoid the user to click too much, then go for onmouseover. However, if you want to absolutely make sure that the user actually intends to see the information you are providing, then an onclick would be better. Note that it’s still better than forcing the user to go to another page for the details and then bringing back to the summary page by back button or a bread crumb.