Glass fibre cable is only really for people who are too lazy to set up the right cable for the voltage they are using. As such it should be expensive. What's wrong with simply using the correct tier of metal wire for the voltage that you are using? This change will encourage more thought in wiring design, while still having the "easy" option. Diamonds are cheap anyway, just compress all your coal and you will have loads.
I think you missed the redesign of wiring as a whole. Glass fiber is no longer a mid-tier, low-loss alternative. It's now the top tier cable, rated for 8192 EU/t. The next step down is HV cable at 2048 EU/t. Therefore, saying "what's wrong with just using the correct tier of metal wire" doesn't work. There is no tier of metal wire to use instead. Glass fiber is "the correct tier" if you need that much throughput.
Now recall that, due to the changes in the e-net, there's no such thing as packets anymore. Instead, all power on a line per tick gets summed up into one figure. Got five HV solars? Then you
need glass fiber. Any other cable will instantly melt. Your only alternative would be multiple parallel HV cables, and that can run you into space issues quickly. (On the other hand, you'll need multiple MFSUs anyway to accept the power...)
Also, keep in mind that people mostly used it because they didn't want the 1 EU per block loss of the HV cable in classic IC2, which could get very expensive very quickly unless you spent a ton of resources on transforming the voltage up to a level that no part in the mod actually output or used, maybe with the exception of silly condensator reactors. And then spent yet more resources on transforming it down on the receiving end.
Currently, cable behavior is unimplemented. All cables can handle any amount of power, all cables have zero loss, no cable will shock you if uninsulated. When this changes at a later date, and we learn how energy loss will be implemented, then we'll be able to judge whether or not going for the top tier is worth the price.