@ elivaughan - I do understand your frustration, and can only say that Juniper is trying to improve their documentation every day. As for those 15 other vendors, I fully expect that the MX can do a lot more networking functions, outside of Port Level Security on DPCE modules, than those 15 vendors can; there are always trade-offs. Especially in the time frame that DPCE modules were the standard bearer, I believe MX was almost never used at access layer of network, and most likely is still not often used there, due primarily price per port. In general it is Juniper devices like EX, where Port Level Security is performed.
This is not an excuse for not documenting this behavior fully, which I am still looking into. I suggested a case so that TAC or someone within Juniper Engg could confirm 100% that this functionality is NOT there with DPCE based cards. I am making a best guess on this.
As well, this functionality was added in Junos 14.1R1, which came out July 2014. This is 1st time I have heard of any complaint in regards to this feature, and it has been out more than 3.5 years. Again, not an excuse, but a fact.
Best Regards.