You can do whatever you want if you are a power user, the tools are there to get around Gatekeeper.

For everyone else it's probably sane to have it, works as a decent filter so someone not tech-savvy don't get hurt by installing malware disguised as an app, one would just need to state incredible features that almost any normal user would like to have, and make them click to install. Gatekeeper diminishes that risk by a lot unless you learn how to bypass it, which requires you having decent skills and probably wouldn't fall for the bullshit that malware apps try to bait people with.