First of all I must say great posts geekypilot and pobjoy
Frequency of training has a lot to do with the SOPs of the school and the experience of the instructor. My instructor took Mad Jocks view that if there was any way to do an exercise of any kind we got airborne. We had only four cancellations in the 43 hours and 7 months it took me for my PPL. We did of course have the advantage of Scottish weather
Other factors are the type of aircraft used, and for the less experienced instructors, where they were trained. (Stand by for flak)
An IMC rating gives me an 80% dispatch rate at an airfield with minimal navaids. Unfortunately the cost of maintenence of IFR aircraft and the drift towards LAA/Light Sport/microlight type aircraft has meant that many more flights now are victims of the weather.
The majority of the sub group A flyers at my field are retired so they can usually just pop out on nice days. Those of us who have to work for a living and fit flying in to the odd day off need the better dispatch reliability of an IMC or IR otherwise we would hardly ever fly.
D.O.