Depends on the person I guess. If you find that your able to play what you want to hear without thinking in terms of what mode your playing. That's great, like I said before I'd ride with that. That is what music is about; theory is just the knowledge of how to understand things and sometimes pull them off.

I think the understanding is very important though. It's like if you wanted to play over a major chord and you wanted a major sound (just an example). Well knowing your scales you know that all the notes in the sound your looking for are in the major scale. You don't need to look for all the right notes, you just know the major scale and you play it. This helps tremendously with improve. The same thing goes with all the others scales, like modes, harmonic minors, and exotic scales. The idea is you know your going to get a certain harmonic sound by playing a certain scale. The same goes with if you come up with a cool lick over a certain chord. By knowing what scale/mode it is in, you will better understand how to bring that sound back whenever. Like in the tabbed example I did in the previous post. It's much easier to know and acknowledge what you did by understanding that part was in the lydian mode. Then you could continue it over the C major chord. The thing is, it's a lot harder to think in terms of how you got a certain sound by just playing a generalized scale over a group of chords. The easier to look at each chord individually and analyze what you did. Maybe at some point you will come up with a chord progression that doesn’t fit in a major/minor sequence. Like Bmaj / Cmaj / Dmaj / D#maj / G#major.. or something, I’m just making it up but you should get the idea. Now if you wanted to harmonize it in a lydian harmony. It would be very easy, opposed to if you were looking for all the right notes over each chord or trying to generalize it with one scale. Just repeat the lick in B lydian, C lydian, D lydian, and so on, or whatever scale you want to use. (*Note: Make sure it fits the chord though.)

I’m not telling you stop what your doing and make sure you know this and that when you play. Go with what you are comfortable with and feel more creativity doing. But knowing this knowledge, maybe there might come a time when you play something that sounds really frickin kool. It’s very good to be able to look back at it and understand what you did, so maybe you can utilize it more freely in the future. Or whatever use you may want out of it. I think this is how styles are created, like satriani uses the lydian scale a lot, well now he knows how to get that sound in a second by just knowing to play a lydian scale over a major chord. And believe me at those speeds you don’t have time to think up every note your going to play, you just know your going to get that sound by playing something in a lydian scale.

As for Slash, I couldn’t speak for him anymore than anyone else. But I did read an article in a guitar magazine (I think) where he talked about the intro to Sweet Child O’Mine. He did address that he used that mode (i can’t remember which one it was, it been awhile since I heard the song and I read the article a long time ago). Anyways, I’m guessing he was aware of it. I’ve read quite a few stuff where certain guitarists like certain modes, Kirk Hammett, Yngwie, and few others. It is a lot easier to break a guitar solo (especially improve) by what scale your going to play, than it is to think up every note. Like I showed you with the chord progression I wrote above. All you got to do is know where the chord changes are and what scale/mode your going to use over that chord. I know harder than it sounds, but that’s a basic mentality of it.