How can I weigh how useful a specific frequency that a network provides is?
It is Mobile Service Provider Specific..

Some service providers May use the frequency and Others may not... It is usually Added so the device can be used with Many Service Providers.

In relation to Missing Out..
The Lower the Frequency, The Further it travels but also the Slower the internet..

The 600Mhz range is most likely Added for Rural area's that get little reception, the speed of the 600Mhz range is a lot Slower than the 800Mhz range, However 800Mhz will Not Travel as Far...

You ultimately are only loosing out on a Slow connection, but also one that travels further..

You will drop in and out of reception slightly More than a device that does support it !