Then I use Sublime Text for big files, quick edits, and the occasional stuff that Atom can't do yet.
But after avoiding Atom initially, trying it again about a week a go was a nice surprise. It has almost any plugin that I used Sublime Text for, and a bunch more. It's good enough for me, and I'm pretty picky.
Atom takes about the same amount of time to start up as RubyMine but it only provides a fraction of the functionality.
The one thing a pure editor has over an IDE is start up time and Atom is not even having that.
Well, Atom does start significantly faster than WebStorm (and it's siblings). On top of that, actually using Atom is much less sluggish than WebStorm or other full-blown IDE's. This is on a MacBook Air though, with tons of apps and at least two browsers open.
That said, I'd probably use an IDE if I do more complex work. For my current web-development needs Atom is fine, and since I mostly work within one project, the startup time is not an issue.
Furthermore, since most of the keyboard shortcuts and interface is similar to Sublime, it's not a big deal to open ST every once in a while for bigger files.
The main reason I'm switching to Atom for now, though, is that I feel it has a stronger future than ST, and I might finally scratch some of my own itches by writing plugins. That's harder for me to do for ST.