I have had friendships like this. Actually I do have a current friendship like this.
I choose to not put an emotional investment into the friendship because I know that the depth I put into what I consider *REAL* friends will not be returned. I don't think she's capable of it.
On the other hand - I have a few new friends that are phenomenal. I think that in the grand scheme of things, these friends will likely be as emotionally connected as I can get - and I will match my emotional investment to theirs.
Some people just don't know how to give.
Some people give too much. I tend to be the 'give too much' type person - I'm learning a healthy balance.
Its all about finding your own level of comfort with those types of people - and accepting them for who they are
In terms of poly vs mono - I prefer one on one situations for my friendships. Or few on one - I don't like groups or lots and lots of people