I think they are important. You may never use them, but at least they are there. People tend to go into alternate states of consciousness when playing hard. Even what some people consider "light" bondage (I'm not really sure what that means, as everyone has a different level of play they consider "rough") and even tying someone up can get heavy, and even a very mild scene can trigger either the Dom or the sub in some cases. Having a Safe Word and Safe Motion (in case you can't speak) is a good idea.
Triggering can occur. On one day a certain action may be the best thing ever, on an other day, it can suddenly feel "not right."
The Safe Word, for many, doesn't necessarily mean "play time is over" it may just mean, "Loosen that" or "Don't do that at that angle, OK?" or even "GET OFFA ME!" and then play can continue as long as both partners are safe, consenting and happy with the scene.
For most people "Stop" or "NO" are not the best Safe Words. When playing hard, Ravishment Play can and often does include using words like this, even when nobody means it. (I'm not talking about dating or casual sex, I'm talking engaging in a scene with someone you know and trust well) Something that you wouldn't say in the heat of passion is probably the best Safe Word.
It also depends on how well the two of you know each other. My Man and I have been together so long that a simple grunt to "Wait" can substitute for a Safe Word, as we've been having sex with each other for decades. He knows the difference between a playing, "OMG stop!" and a
real cry for stop in the play for a moment or just an adjustment to what we are doing.