My main concern is security (malware, adware, etc.). Would you say it is key for me to upgrade for this purpose alone? I'm pretty happy with Mavericks, but am aware that their last security update for 10.9.5 is likely their last. I'm not exactly keen on the aesthetics of Yosemite, but security would be the most significant/deciding factor for me. Any thoughts would be greatly appreciated. Cheers!