There's really no need for the iOS Simulator to steal the focus away from Xcode when building your project. Wouldn't it be great if you could keep working in Xcode uninterrupted?

Note: the following setting will remove the backgrounded application from OS X application switcher.

Add a Launch Service Key called LSBackgroundOnly to Simulator's Info.plist. That is, append

<key>LSBackgroundOnly</key>
<string>True</string>

to

/Applications/Xcode.app/Contents/Developer/Applications/Simulator.app/Contents/Info.plist

This obviously won't improve your workflow when crammed on a laptop, since now Simulator.app won't be shown on CMD-TAB and you'd have to otherwise bring the Simulator in front. But otherwise, keeping Simulator in background makes for a better default behaviour.

This customization isn't Xcode-specific and could be beneficial whenever your work involves another process whose window-focus you deem inessential. Thanks to Kevin Chen on Ask Different for this nugget.