I'm pretty sure a lot of couples would not agree with me on this, but as much as I want to find reasons why engagement rings are not necessary, it's kind of hard.
And I don't want to assert that such accessories are a must either, because at the end of the day, it will all depend on the couple. But personally though, I would like to quote Beyonce:
"If you liked it, then you shoulda put a ring on it"
Even though engagement rings can be considered as materialistic representations of a superficially perfect love affair, they actually represent a lot of things. For one, it's the time bomb that reminds the couple how far out their big day is. It's like the moment the guy slips the ring into the girl's finger, the real planning starts. I know there are couples who still don't have a plan even if they get engaged, but they're probably part of the exception.
Second, engagement rings are a sign that the guy is ready, not only to spend the rest of his life with the girl, but also to take responsibility of what the future holds for them . Now unless the girl is willing to pay for everything and provide a sustainable living for her future family, engagements rings are not really necessary. But that's unlikely.
Lastly, engagement rings can take regular boyfriend-girlfriend relationships up another notch. They set a period to rethink the whole relationship and see whether the couple are really meant to take the big leap. They separate the couple from other couples who are "taking their time". Trust me, I know some couples who are "engaged", but because they don't have an engagement story, I couldn't tell if they're settling down any time soon. When you have a ring on, you know that your shit just got serious.
All I'm saying is, engagement rings, or whatever accessory that represents your engagement are necessary---it's the expensive, unrealistic ones that aren't.