Imagine a Thief firing off Sneak Attack + Trick Attack + Shark Bite. At most, you are looking at three macros using the FFXI system, though this combo can be initiated with a single macro.
Now imagine typing "/Sn" enter "/Tr" enter "/Sha" enter for that same combo. Assuming there are no typos, you're still looking at significantly more input than is necessary.
You wouldn't use that feature for complicated, frequently use ability sets though. This would be more for the situational ability that you don't really need or want a macro for, or maybe the macro you wrote isn't working perfectly in this situation. Maybe you need to do a SATA WS for a SC, but your SC partner died unexpectedly, and you need to switch to a new SC partner, but you don't have a macro for that SATA WS. In that case, /sn enter /tr enter /sha enter may be preferable to editing your macro mid battle.
Especially when /waits are required for commands, and I can type out three characters in less than a second easily.
But I want to point out that macros are just another way of inputting commands. Before macros, and even still today, games have managed to get by with just regular controllers. You still get characters that can execute dozens of actions, but with button combos rather than keystrokes. There's no reason you can't write a macro that's executed with a "Hold R, Press Right + A" rather than a "Press F10." And you might think that sounds more complicated, but to someone who is used to a controller, they can often input those same commands as fast or faster than someone who is used to a keyboard, purely by virtue of having practiced on controllers for so long. You have a lot of gamers who can execute a twelve button combo faster than they can find the F6 key, or press Ctrl Y.
And I guess the issue moreover, is that I think there are a lot of players who want this kind of option.
Mouse gestures are unlikely as this would be difficult to port from the PS3.
It's really nothing that can't be done with the D stick, but I'm not opposed to additional hardware like touchpads, or possibly even the Wiimote ripoff.
And once again you are more likely to make an error at a critical moment in the battle (like during the middle of a skillchain) trying to execute a specific mouse move than simply pressing ALT+1.
Well, that's kind of the idea really. It keeps you on your toes, adds difficulty to the game, refines motor skills, etc.
Button combos might also be difficult to port to a keyboard from a controller, and aren't FFXI macros simplified button combos anyway?
They shouldn't be at all difficult to port. There's no reason why they would be, and really no reason why you'd need to port them in the first place. And to your question, it's a tradeoff. The command input is simplified in exchange for a more complicated interface device. You may think a button combo is cumbersome, but to many people, a keyboard where a controller will do is more cumbersome.
I think it's another example of an easy to implement option. It's not so difficult to add to the game and it would take nothing away from the people who choose not to use it.