Yeah, that's my policy in a nutshell. For more detail, read bellow.
Why I may de-friend on LJ
 De-friending does not mean I hate/dislike you. 99% of the time it means I'm not reading your LJ. This may be because the majority of your lj's content consists of:
- birthday wishes
- RL (for me LJ = primarily fandom)
- fic posting or icon posting
- a fandom I don't/no longer follow
Those are also the reasons I may spend a lot of time lurking on your lj - don't have much to contribute to the above esp if I didn't friend your lj initially for fic/icons.
It may also be 'cause we haven't been connecting much:
 We haven't commented on each other's ljs in a while (weeks/months)
 I have been skimming your posts
Whether I friend/defriend/do not friend your lj, you are always free to friend/defriend.
In many cases I may not have peeps listed as 'friends' but I still consider them to be seriously awesome.
How I use the 'friending' function on LJ
- My flist is a reading list. I try not to keep anyone on it whose posts I end up skimming/skipping (and we don't connect via comments)
- I will not keep you friended and filter you out because that just seems worse to me than defriending.
- If we don't trade comments for weeks/months together, I will assume that we're skimming/skipping each others posts, are no longer connecting and may defriend.
- If the majority of of our lj communication is spent with me commenting on your lj, I will assume you aren't all that interested in what I post and me keeping you on 'friend' status will depend on how interested I am in your lj posts.
- If we (re)connect and I find myself reading your lj (again) in future, I will (re)friend you.
Everyone has their own way of doing things and this is mine.
Edit: On re-reading...*headdesk* Definitely suck at putting things into words :(