Because I want to run more than 100 instances of a webserver per mongos process, but I can't because it causes the connections per server to go over 20K.
Right now my application stack is woefully under utilized due to this completely arbitrary decision on their part not to allow the end user to set the connection limit. They've even admitted that they just picked that number four years ago and haven't looked at it since.
Just like they limit replica sets to 12 members maximum. What if I have higher read requirements than that limitation allows? Well, too bad.
I think this points at a fundamental issue with MongoDB at the design level - they don't allow end users to make any decisions about how to configure the product, even if those decisions might turn out bad.
Every Enterprise-level DB allows end user tuning of parameters, so clearly MongoDB isn't Enterprise-level.