I wouldn't say there is no value. I changed mine to 5 1/2" components. I enjoy the rear stage, but recognize the front stage is the most important. I can hear the rears the way I have mine set up and enjoy the fill they provide. Obviously, more quality is better than the stock that was in there.
I wouldn't say it's as simple as assigning them to have "no value."
There are exceptions, such as if you listen to club music, where the sound isn't mixed to be in a front stage listening setup.
Other than that, the better the front stage gets the less you need a rear stage. I have a nice set of amped 6x9s in my car, but as my speakers up front have gotten better I've faded more and more away from them. Now I have them turned completely off and it's the best sound I've ever had.
I will definitely stand by that you would get more value out of upgrading the sail speakers than the rears.
One of the big issues with car audio is that car manufacturers have ruined people's perception of what it should sound like. They put in setups that are balanced front and rear, which only makes sense for having sound for the passengers. People get used to having zero imaging in a car environment and learn to like "bad sound" that is just based on filling up the vehicle with noise.
Think about it this way, you would never turn your back on something to listen to it. Our ears are built to listen predominantly to what's in front of us. When you put sound from the same sources (same instruments or vocals) both in front and behind you, it doesn't sound like it's coming from anywhere. Again, it's just noise. If you build a system right it can sound like the band is sitting in front of you.
Now I am going to keep the rears going forward, but that's because I'm going to do some complex signal processing to create a simulated larger listening space.