Macros and global replace are substitutes, sure, but they're
not at all the same thing. And while I'm no vim wizard, I have used it as my main editor for 4+ years, and have a 1500 line vimrc and plenty of plugins (including my own), so you don't get to assume I don't grok the vim way of thinking :)
I've heard the "macros is the same" sentiment many times, and usually it's from people who have never had much experience with multiple cursors, at least not in the situations where it's useful, and therefore don't realize how much more convenient they are in many every-day situations.
In fact, many of the people arguing against multi-cursors have been convinced after trying them that they are better.
Please, read my linked comment for an all-too-common situation.
The basic problem with macros and global replace is that they don't happen immediately and don't give you feedback. I don't know any vim user who hasn't had the distinct pleasure of needing 3 tries to record a macro before they got it right.
What's more, if I record a macro and run it, then realize after it that I want to do a small addition, I have to go through all the "setup" steps again - go back to the start, re-record, add the logical of finding the next block, and so on. I can't just visually see the situation and say "oh yeah, why don't I add this little tweak".
Seriously, take a very common example: having 3 if's one after the other, where you want to add a line at the start of each if block. Doing this with multiple cursors is a breeze - 3 buttons and you get 3 cursors, then write your code. And if, after writing it, you realize you actually want to add another line, you just keep typing. It's literally the same as typing.
Anyway, I've rambled enough on this topic many times. Please, just give them a try - I care a lot about text editors, I love this field, and I honestly think multiple cursors is the best innovation to happen in text editing in many years. And yet it's so often overlooked.