There is nothing to do with revenue model or anything like that. That's shortsighted.
All forums are pretty much built that way (which allows members who fall off and no longer visit to not be bombarded with email). E.g. recent Ross discussion received 91 replies in one day.
We have done a few things to add value, however:
1. We provide the text of the post in the message that is sent to you (both new messages, replies, and PM's) - we had to invest into this feature and develop it ourselves. Per your suggestion this would technically decrease the number of visitors clicking from email.
2. We have created an override to that for some moderators who are interested in receiving all emails regardless of the one-email native limit. We turned it on for all mods by default but had to turn it off since it was sending them way too much email.
Finally, we do have to watch how much email we send - it is a sensitive issue as half our users use gmail and we don't want to flood their system every time someone replies in a multi-thousand "Calling all Applicants" threads - just imagine thousands of messages going out every time someone replies... and now multiply that times the number of members we have: 240,000. You get some astronomical figures.
Anyway, I have enabled the feature so you will get every reply for EVERY thread you are subscribed to. Let me know when you are overwhelmed
Great service! Thanks. When I start unsubscribing you'll know I've exhausted my reading capacity! Thanks again.