Some interesting points here.........
The "to spar or not spar" dichotomy never really raised its head when I first started learning IMA. What
wasimportant (like Allen sed) was that what ever you did had to be done a certain way; so that maintaining sunken Chi and still mind and all that jazz was paramount. We where given some step by step "ritualized" exercises (that gradually increased in intensity) that gave focus to operating from a particular mind set. In the three main schools I practiced with, this was the go. If you didn't keep your mental S%*t together, you started breathing wrong and lacked effectiveness in pretty much all aspects of your game.
The progression of working with partners went something like:
1) Test your postures in a controlled manner
2) Test your postures in Pushing hands in a controlled manner
3) Test your applications in a controlled manner (one step stuff)
4) Work on free pushing hands
5) work on structured Ta Lu in a controlled manner (introducing moving step)
6) Work on free Ta Lu
7) Work on structured San Shou (ever increasing intensity of attacks) bringing together all previously learnt skills from the levels above.
8) Free Spar
Each stage built on the last and required the mind set to be tested to a great degree as we went up the scale.
I think the to spar or not spar question is analogous to practicing spiritual endeavors in a temple as a monk or as a lay person amongst "average folk". Neither is right or wrong, but the latter will test you in a very concrete and down to earth way. You know....."in the world, but not of it"
When I hit Bagua hard, I had this back ground to build on; so the transition was I believe, easier to work with.