Anyway, XML is better than plain text. But there are no plaintext sources in most commercial websites.
If there's the need to parse the html of a site and extract the infromation (regexing)... It's just too much. And normal users won't be able to do that.
On the other hand, if it was usual to offer XML output of the billboards or any other boards and tables of services, the world would be much better.
The idea is all right, but it's really difficult to achieve a system that would work well easily.
For example:
If the public transport companies would have XML oputput I would code A system that would tell me exactly when to take a train if I want to be at a certain time in a certain place and I could add that to my digital scheduler without having to look at the timetables myself and calculate the time I need to get to some place.
Unluckily XML (or say plain text) is very rare in these cases. Neither companies nor webmasters care about standardised outputs that may interact way better with many other services.