I really think that there are a lot of people out there who do not understand that RTT is not the same thing as two times the one-way latency. In other words if you measure (A-to-B + B-to-A) / 2 then you have failed to learn anything about one-way latency on either path A-to-B or B-to-A. Yet that's precisely what people are doing when they measure RTT and then assume that RTT/2 is equal to the one-way latency A-to-B.
What you've said is true, but there are times when RTT measurements can be useful. If you are making measurements on your own backbone and know that paths are either symmetric or have a well-know latency ratio, then RTT might not be a bad measurement. Moreover, though this isn't really what the original question regarding "latency" measures was about, there are times when you don't care much about the absolute value of the RTT, but rather you're looking for latency spikes (i.e. RTT(t) - RTT(t-1) > 20ms ) as a crude warning of either a topology change (i.e. my traffic that normally goes direct from Boston to NYC is now going via Chicago) or of some sort of link/router issues. When this happens, it might not tell you which path or device is having a problem, but it is an easy monitor to set up to tell you that something has changed... Eric :)