"The triple point is a definition rather than a measurement: the three states of water can be calculated to exist at 0.1°C and a partial vapour pressure of 611.73 pascals – in kelvin, 273.16K."
Err, you missed a zero, it's 0.01 celcius. Everyone knows that, yet even QI got it wrong once.
And I'm pretty sure it's not calculated to be anything, the triple point of any pure substance is just whatever it happens to be, but it is fixed and easily reproducible which makes it a very good reference point.
Here's a thought:
The newton is defined in terms of the kilogram, which is a problem unit to define. Triple points don't just have a precise temperature, they also have a precise pressure. Could that be used to define the newton, and hence the kilogram?