Many people do use them for that, and that's probably why KEF offers a single one for a bit more than half the price of a pair.
I think they're so good it's a little overkill on sound quality for most movies, though there's BluRay disk like the Eric Clapton Crossroads events that help the LS50's really sing.
KEF R500, R200C, R100, R50, SVS SB2000 sub, NAD T758v3 AVR, LG UP875 4K BlueRay, Samsung UN55JU6500F 4K LED TV, KEF M500 headphones, Sony CDP-CE500 CD changer. Office: NAD M10, KEF LS50, KEF Kube 10b, Pro-Ject RPM1 TT w/ Ortofon 2M Red. Guest room: Hafler 300 L-R, KEF Reference Model 100 center, KEF iQ10 surrounds, modded M&K V-90 sub, Bluesound Vault 2.