Combining monitoring data from multiple locations would give a more accurate response time overall. But as a site owner, that number is not very useful because it's not actionable. For example, if response times to my site become slow from say, Dallas or Ireland, there's is nothing I can do about it other than be aware that it's happening.
Monitoring from a single location is interesting because it gives a metric for how well my application is performing.
Certainly, but I would say that there are very few Internet startups that cater to a single (or even a few) geographical locations. Your numbers currently indicate quality of service for customers in the same hosting block as Amazon EC2.
If you have enough monitors and spread them out well (which shouldn't be too hard, since you could just sign up for Perl/PHP enabled hosting accounts in different locations), you can still diagnose where the slow geographical regions are.
In short, having more data can only give you a more accurate picture, depending on how you analyze it.
Monitoring from a single location is interesting because it gives a metric for how well my application is performing.