Why bother wasting memory on something you dont need?
Your probably wasting much more through the alarm, that is why. Seeing as the alarm will count down on each step that occurs. Thus the alarm will be burning away additional CPU calls to do exactly that. Memory isn't a huge issue, especially when talking about a Boolean value. in C they only take up 8 bits(or 1 byte) Are you seriously going to aim to save 8 bits when most users(based on steams reports) have 4 gigs of RAM, thus 3.2e+10 bits~.
Saving the memory makes no sense in the context of current hardware. Doing it efficiently however might matter more especially if you are building a CPU intensive game.
Ontop of that you've just cleared one alarm from being used as a Boolean to it being available as something else, as GameMaker still limits you on the amount of Alarms you may have(unless has changed recently, I have not updated some time).
Also I don't think the alarm will take up space in the actual compiled game if it is not used. I haven't tested this but I assume the programmers behind GameMaker are smart enough to not compile unused Alarm events.
And as my last point. If the user ever decides to move to C# with XNA for example, knowing how to do this using an actual Boolean will prove benificial as the alarm event will no longer be around unless it's built up from scratch.