If you do nothing more than a scoped search-and-replace then regexp may be more practical. But if the edit action also involve moving cursors around after selection, or when some match should be skipped then it takes less effort to use multiple cursors to edit than trying to comes up with regexp and edit command that accomplished what you need.
I wish vim had a visual regex multi-replace tool, like the visual search tool. So matches would select as you typed, and changes would update in real time too...
:10,20s/poorly_named_var/nice_var/g