So, most of this thread has been about whether sparring is effective training or not. Most agree, that it is at least an important part of training.
I think though, that how you approach sparring has a huge effect on how effective it is.
I have been training danzan ryu jujitsu for about 20 years. I have rolled with dzr guys, bjj guys, judo guys, mma guys and a few wrestlers. (learned a lot from all of them) I recently started training in Shotokan karate. The distance, the stance, the techniques are totally different. We do a lot of sparring. Now, if I approach these sparring sessions from the view point of: I need to win... Then I take these guys down with a sweep or throw and submit or pound them on the ground. I win. But, I don't learn much. I already know I can throw guys, I can submit guys... especially when they don't train grappling arts. However, if I can swallow my pride, and try this karate style sparring at the new distance, with the new techniques, I lose the sparring sessions... but I learn things. Then, I practice those things and get better.
The sensei will show us new combinations, new set ups, different approaches... Some students, either try it once in sparring, get countered and go back to their winning stuff or they never try the new stuff at all. They want to win. Its the same thing as grappling non grapplers. (or punching non punchers)
I think that in order for sparring to be most effective, you need to try to do the things you are not good at, more than the things you are. That way you are expanding the number things you can be effective with against resistance. This is where the humility thing comes in. In training this way, you get beat by people who should not beat you. However, in return, you learn to apply techniques that you couldn't use before. At least, for me, this is what makes sparring most effective.