Hi Ocho,

The reason why I said mountain cranks are supposed to be harder to flex is due to the needs of mountain bike riders who put their bikes through a lot more stress, abuse and torquing in 6 to 8 or even 10 foot drop offs that if the cranks constantly flexes, then eventually it will break. There will be continual liability issues with Shimano and that is not good. Mountain bikers are finnicky people you know. But that is not to say that you can not flex it. The most likely scenario and I had experienced it myself twice already and had almost forgotten cause I now have a Philwood is the short lifespan of the LX bottom bracket. It just died too quickly and developed a very slight noticeable play on the spindle. This will give you a flexing feeling when you pounce on the pedal. I went through 2 LX BB before I upgraded to XT. That went a bit longer before it too developed the same play.

Usually higher grade mountain components like XT and XTR are designed to be used and last only on race day or days only. They are designed to be light weight and not necessarily durable. Most tourers would suggest that you skip the mountain cranks and go with the Sugino XD 500T which is a commonly used crankset with the touring crowd and then customize the gear inches to your liking. I had used it and liked it on my previous road touring bike.
I use an old XT crankset square taper mated to a Philwood on a 26 inch touring bike now with custom chain rings and have never been happier with the setup.