e10s will bring some security and stability benefits. If one webpage crashes, all webpages crash, but you can click to reload. Additionally, this enables sandboxing, so that code from webpages can be run with lower levels of access. Sure, it is not perfect, but it is a large improvement from the status quo.

(Even process-per-tab is not perfect for security: a compromised page from one origin could open an iframe to another origin.)