Z, like you say, it does eventually get to you and makes it difficult to answer each PM you receive with a positive attitude. Treat others as you wish to be treated, right? If they don't reciprocate the courtesy and time it takes you to respond, then like you say it's a good screening tool and in the end you will probably save yourself a lot of hassle!
It's a shame and really sad, but not everyone has manners...a lot of people are great, but sometimes the small few can leave a bad taste I don't think everyone wants what they want when they want it, and they won't accept anything else. A lot of real members are good people, so just try to think of the positive ones and hang in there!
If you need a break, you can always fly west, doors always open!
Cheers