Honestly, I bought the MSI GTX 970 ($350 flavor) without a backplate and I regret it (the lack of a backplate, not the card!). The card sags, and does not feel solid at all. I'd recommend buying a card with a backplate. Personally I am now looking into 3rd party solutions as I fear for the longevity of the card, and they're going to run me like 50 bucks. I'm debating using the 970 false advertising scandal just to get a refund and buy another 970 with a backplate. I know, I'm horrible. :x
Performance-wise it shouldn't bother anything, but they look better and are much MUCH sturdier. To me personally, I wish I returned it as soon as I saw the sag post installation. The longer I look at it the more I hate it. But I was lucky enough to get a really well manufactured card (dead silent, no whine and very low temps) so I can't complain too much.
Ultimately, if you're spending that much money on a card it shouldn't look or feel cheap, IMO. Spring for the backplate.