For the most part, as long as you take off the backplate before returning the card for warrantied issues, backplates expressly do not void your warranty so I don't know where all this chatter is coming from. Third party card manufacturers sell backplates specifically for the cards they manufacture... is it all some sort of elaborate trap for the unwary to void their warranties and let EVGA off the hook? No, of course it isn't, and backplate installation is about the simplest procedure one can undertake with a screwdriver.
And yes, the major advantage of adding a backplate is cosmetic, but the secondary benefits are actually functional. It acts as a heatsink for the rear portion of the onboard vram, which can lower temperatures a few degrees. It adds structural rigidity to the card itself and can act as a physical barrier to impact damage, say when you drop a cpu cooler mounting screw onto your card during an upgrade or cleaning.
Ultimately its a matter of personal taste and what you value enough to spend money on. If you run your cards at stock frequency and don't have a side window on your case, or generally could care less about aesthetics in your computer case then its probably not for you. If you're looking for a pure value proposition in your component purchases, again it's probably not the right choice. If you like the way they look, plan to crank every ounce of power out of your gpus through overclocking, and have a bit more leeway with your budget then yes, they're a fine choice.
I personally love the look, but I tend to care more about my computers internal aesthetics than a lot of people. It's a hobby for me and everything I do with it isn't necessarily a value added proposition. Your mileage may vary.
disclaimer: no, it won't get you another 100mhz on your overclock, but it will help slightly in temperature management. If you're really serious about overclocking you'd want a custom water loop at least, however that's a several hundred dollar investment at best, as opposed to 30 bucks for a few degrees and some bling. As you may already know, the price per performance ratio gets really skewed at the very high end of pc components.