Well, I do understand the point of an engagement ring - what I am completely flummoxed by is the idea that you should spend a huge chunk of money on one.
I got my wife an eternity ring as an engagement ring - not because of any rules but because we both liked it and it wasn't very expensive. Been married 27 years so far so it seems to have worked! ;-)
You are implying then that the point is to pay a lot of money and your future wife should be happy that you are paying a lot of money for a useless thing, just to show you did it. This is a bad ground for a relationship IMHO. Because otherwise the physical qualities (how it looks) is the whole point.
A nice engagement ring does not require a diamond though. It does not require 3 months of salary.
So what _is_ the point of an engagement ring? To show love? I can do that without a ring. To show I'm taken? I've got another ring for that. We both do.