As mentioned, there is a limitation to TCP-based speed tests - TCP throughput is very sensitive to packet losses, particularly during slow-start, in addition to requiring end-host tuning (as an exercise, try running speedtest.net on a high bandwidth connection). You could use something called "available bandwidth", which is kind of like the "leftover capacity" on your path - I've been using this tool called pathload2: http://www.measurementlab.net/measurement-lab-tools#pathload2
At the moment the market is competitive and clients are getting
various different offers from different competitors.Thus you find them
enquiring about speeds hence need to check on the speeds.
onsite engineers too need to be able to test and provide accurate
results.thus need for a tool that can provide accurate results.
Regards,
Shake Righa
see a comment by me in the previously mentioned thread: "swedish isps
do this for their customers, perhaps one/some of them could present at
a *NOG as to how they do what they do?"
-chris