It's really only necessary if you develop a painful hot spot at the top of your head, where the leather padding on the stock headband forms a seam. Even after re-adjusting my LCD-X (and LCD-XC... and LCD-3), the pain would persist.
I purchased a Lohb comfort strap for my LCDs and it does the job, but I didn't like the aesthetics and I was also worried about the elastic stretching out over time.
The carbon fiber headbands are expensive, but I think they are worth it more or less. I think they'd be more fairly priced at $100-150, but it is what it is.
Does it make a difference in comfort? Yes. Depending on your head size, I'd say that you should expect to increase the notch by one when using the carbon fiber band over the stock band. If you are near the end of the notches with the stock rods, you will definitely need to buy the extended rods as well.
I know someone else mentioned providing comfort impressions, but there isn't much to say... Again, it's more comfortable. The headphones become more flexible because of the carbon fiber. There isn't any hot spot with the fixed headband. (Though I'm using the OG carbon fiber band with the leather strap issue on my LCD-X and I have it set at a certain length so that a hot spot will not develop.)
If you rest the headphones on a Woo Audio stand, you don't have to worry about the memory foam of the stock headband compressing and you don't have to worry about the leather developing an indentation when using the suspension headband.
