Most of the readership on HR doesn't. But most consumers want their phones to "just work" - unless they personally feel the pain of unpatched vulnerabilities, they expect that whatever is running on their phone continues to work. So it seems that the carriers have decided not to risk keeping their customers up to date (not wanting to take on any extra tech support). Carriers don't generally care what you're running on your phone, but since many handsets have locked bootloaders (and/or few people maintaining alternative ROMs), choosing a Nexus device means being able to do what you want, which could include being more secure.