Take the "friendship" of a man and woman who have no professed attachment to each other, made bittersweet by the hidden love of one for the other-
... Bluntly, this is bad.
1) Emotional input is not equal. Being romantically (but secretly!) interested in another human being consumes a lot of energy. This can lead to exhaustion, exasperation/feelings of neglect, and confusion on part of the "lovee." (a better word?)
2) Disproportionate attachment, where one friend needs the friend more. A note on word choice- a feeling of deep "need" is generally more characteristic of the connection between lovers than that between friends.
3) This friendship can lead to the "stolen single-ness syndrome." Imagine the message sent by a friend-couple that arrives at a party... most persons will assume they're an actual couple unless they bother asking around. If a girl is busy hanging out with her best guy friend (who she secretly adores), it's possible she'll miss out on the guy who might actually be interested.