Right. I've found the dair spike is only really necessary when you really need to get a stock off of them quick. (you're behind) You get that any higher up you're better off using any other kind of edgeguarding.
A dair spike is only necessary if you find yourself in a situation where you think it will work. Never limit your thinking to certain options which create habits. Watch Marth videos, check out this link :
Marth Videos to get a better understanding of how and when dairs are used. You may even learn other things as well.
Please read a bit into:
If you do not know what these words mean, short hop, fastfall, l-cancel, or shffl I recommend watching the 3 advanced how to play videos by Wak. First link is here:
Advanced how to play Part 1
There is a little part there about doing the ledgehop dair correctly, as well as cetain questions you may have later.
Beginner Guidlines by Blue
Basic strategies with Marth you might want to read about and possibly implement into your game. Developed by Emblem Lord.
Approaching with Marth
Edit: Fixed link. Thanks Jman.
The reason why I posted all the links was because of how Dark Oni replied to his own thread, made me think he would have more questions later.