Back on-topic I think @natebeaty sums it up nicely, but for power users some editing tasks require the flexibility to define a regex search as part of a compound, possibly repetitive, operation across a large file. I could give examples, but those who've requested this and chimed in on this subject know where I'm coming from. Most of them I would assume - and myself included - come from traditional power-user text editors like Emacs and VI. I myself came originally from Brief, via Crisp, Vim and Emacs.
Macros that can record and script anything the editor can do are crucial and having a macro system that can script many things (ie. TextCommands) but not all editor operations which can affect the buffer or view is both confusing and frustrating. As a result, like @natebeaty I just avoid using macros for anything but the most trivial caret and text manipulation convenience functions.
In Crisp, I would routinely record a macro that would find a regex, then move the cursor from the found string, do some edits, go down and to the beginning of the next line. I'd then hit F8 (playback macro) multiple times to perform the same operation on repeat across the file. Or I could hit Ctrl+Y (repeat), then type 20 and hit F8, which would repeat the macro playback 20 times. Or I could hit Ctrl+Y+0 (repeat to EOF) and hit F8, which would repeat the macro playback until the Find operation would cause a wrap. You get the idea.
In truth, I would use Replace rarely in this context, but Find often. For the above workflow, Sublime misses the ability to record & script Find/Replace, but also a 'repeat' function. For simpler operations, I've found ways of getting to the same place with multi-cursors, but it's slow, more restrictive and inappropriate with particularly large files. You want see whats being changed linearly and in a stepwise fashion. You want to 'pause and resume' as you go, making other edits in-between. I don't argue this as an alternative to multi-cursors, which are awesome, but they're not suitable in every situation.
One improvement that could help a lot is the ability to define a regex match parameter for Ctrl+D style matching. Find & Replace allows find next and highlight all, but not selective highlighting. It still means we're in multi-cursor land (with the same disadvantages outlined above) but it would get me closer to solving many of the situations where I'd be having to fire up another editor.
It's probably fair to say that I would use this sort of thing rarely when purely coding, but often when doing transformations of large text files laid out in a particular way, like SQL dumps, tabulated text and CSV files. The ability to compound editing procedures into macros and play them back or repeat them is still essential for at least a part of my text-editing life I'm afraid, and feels like a real weakness of Sublime.