Very interesting topic; I've been thinking about that as well for some time, so here's an occasion to let out some thoughts and questions I came up with.
First, to answer your question on if VGM can live on its own outside a game, I'd say a clear "yes"! But I'd add a nuance: I consider there are two reasons why someone can enjoy VGM; either because listening to it brings back memories (nostalgia) or because he actually enjoy the music, as GoldfishX said.. It can also be both reasons at the same time, but enjoying the music without having played the game it's from is a definitive proof to your question. As for myself, the majority of the music I listen to is from games I never played, and I play less and less as time goes by, like some others, so this tendency will increase.
On the other hand, there are people who only like music from games they played. I understand it, but find that it's a sign of a lack of open-mindness. But when some of these people also go as far as listening only to VGM and not enjoying any other category of music that exists, that's extreme! In my case, I listen to many other things, from mainstream/pop/rock, to new age, to symphonic, to jazz, to pretty much anything. I'm not saying this makes me better, but it's hard for me to conceive those people actually are music fans, because they are not. Maybe they don't want to in the first place? Anyway, everybody can be happy with what they want. Let it be...
Now, I'd like to reverse this and talk about how people who never got into video games see VGM. RyuFAN mentioned that it's hard to include VGM in a discussion with someone who's not into it, and I'm also feeling the same. Sure, you can still mention it, but each time I try to, I feel insecure. I have this conception that most people have a bad opinion on video games and the people who play them (see "nerds"), and by saying you listen to VGM, I guess you'd be put in the same category, which isn't right because you can enjoy the music while not playing the games as we said. If you have a strong self-confidence, you can still go ahead and live with it, but unfortunately, I'm lacking this quality and am stuck. But I'm still thinking, and these days, it's about what actually is of interest in VGM, musically-wise. This would give me arguments to explain to someone why it can be enjoyable. There are so many possible reasons for someone to enjoy it that I'm often getting lost. Also, not eberybody enjoys the same kind of VGM, so that adds to the difficulty of coming up with a clear answer.
The people who played to video games in the past make up the huge majority of the VGM fans. Even though some can listen to music that they never played the game of, and then believe it's because they actually like that kind of music, while it's true they have that taste, it doesn't come out of nowhere; their ears have been used to that kind of music and sound while playing games or listening to VGM. That's why, for example, an old lady of 65 years old, who's a music listener, won't go out and buy the soundtrack of Raiden. So, what defines musical tastes? I guess it falls down to what you have been exposed to and also in which context, so it can create an emotional relation, sometimes subconsciously. So yeah, it's possible to enjoy the technical aspects, but I believe there's always an element of how the music makes us feel to explain our tastes. For VGM fans, it's mostly because of their gaming past, but you could take someone that was raised in New Orleans many years ago and he might enjoy jazz from having heard it so often. Another person might not like it though, even with the exposure, so I guess the answer is more complex. There's also mass influence, especially in mainstream/pop music. You see so many people who listen only to this, that I'm wondering how many actually do it because they like the music and not for the conformity confort it can bring. Yes, the music can be good, but there's absolutely nothing else that is enjoyable to them? Wow, that fascinates me! I actually consider those groups as "sleepers" or "sailing ships", cannot thinking by themselves and going to where the external influence guides them. This doesn't apply only to mainstream fans, but to anybody who have very small taste limits, even in VGM like I said. Still, listening to many things can lead into a lack of focus on the things that we like the most and then start to waste time. What's better to do then? I guess a compromise between the two, as always...
So, what would you say to convince someone that VGM is actually of interest, even if he never played games? Starting by saying that it's mostly instrumental and that it holds some unique styles that probably won't ever be heard anywhere else could be a good start. It's very diverse too, although there's much of it that is electronic music. Many pieces have a fast tempo too, some people don't like that. I know there are other good points that can be used, so thrown in your idea!
How ya doing, buddy?