It's sort of like saying "the problem with cars is that they don't make effective use of the horse".
i3 doesn't do anything to prevent you using the mouse for whatever application you like.
For extra stuff i'd like using the wheel up/down on a title bar to minimize/restore a zone, drag-dropping a window over another window to create a zone made out of tab-like title bars, ctrl+clicking multiple titlebars to select multiple windows (so that, e.g, drag-dropping them to another zone would put all of them as tabbed windows in there or being able to close all of them either via a shortcut or via a popup menu command, and/or commands to merge them in one tabbed zone, or split tabbed zone(s) to individual zones), right clicking on a zone separator to offer a popup with commands to split/merge the zone, move the splitter at some specific percentage, etc, drag-dropping one or more windows at the edge of the screen to create a new zone (or set of zones for multiple windows).
There are so many things you can add with a mouse, especially if instead of thinking of it as a car vs horse you think of it as what it actually is: an additional input device that is right next to your keyboard and unlike the keyboard it provides precise analog input for 2D motion.
As a starting point you can check what pretty much every IDE with support for tiling does (e.g. the recent versions of Visual Studio - not VSCore, the real one).
It's really quite a powerful and easily configurable tool, so much so that I consider it to be an absolutely indispensable part of my day-to-day computing experience (working on a Mac, for example, is just depressing in comparison).
I know other window managers allow you to use focus-follow-mouse, but I haven't found any that make it nearly as effective as i3.