Marriage is for the individual people to decide how much importance they put on the unity (or "sanctity") thereof.
So if marriage is important to you and your partner, you'd want to celebrate AND make the world (guava-mint) know that you're legally "binded" together in union.
If you prefer to stay together as if married and had a ceremony maybe (wiccan perhaps, but even they do get the legal documents because of the nice perks you get sometimes when married legally) then it's up to you and your partner to decide that.
Lucky you edited your post above to add that disclaimer...
I think you're right in that the importance of marriage to you depends on how how you see the sanctity of marriage.
I do see marriage as being more of a commitment than a monogamous comitted non-marriage relationship - but that's because I take the marriage vows very seriously (and would expect my husband to feel the same). So while I know you could cheat in either situation, the way I see marriage is that it is even worse to cheat in a marriage than in a committed non-marriage. (I realise not everyone sees marriage that way, though).
And partly that's because marriage is a public commitment in front of the law, your friends and family, and if you're religious, your god. So I guess i'm defining marriage as exactly that, a public commitment in front of ... etc.
A private commitment is, of course, still a commitment and shouldn't be broken! Again, it all comes down to how you see the commitment; I'd be worried if my partner made a privae commitment but wouldn't make a public one, because to me it would seem like he's less committed.
Anyway, hope that's clear - my thoughts are a bit woolly this time of night. and hopefully it's doily-free as well
Edit: I guess I also see marriage as being more difficult to get out of than a committed non-marriage - so you're forced to work on it, to stick together and work out your problems together rather than just giving up. (Although I do believe in divorce, because sometimes you can't work things out. But you should have to try.)
I guess what it boils down to is that I see marriage as a greater commitment than a co-habiting committed relationship.