x265 is still a very young project. Yes, you get a considerably smaller file (depending on settings), but storage space is cheap, and as others have mentioned x265 takes lots of time and computing power to do its work.
I'm satisfied to stick with x264 for archiving. The "slower" setting, combined with a CQ setting of 18 for SD, gives me the quality I want with reasonable encoding times. I don't normally encode HD discs (unless I want one on my iPhone); I rip them with MakeMKV and store them as native sources.