I'm not bilingual. I'm not very fluent on some languages either but I dearly cherish my ability to speak somewhat understandable more than one language. I understand more than I can express, for some reason, though.

I like different languages because I can deal with different things in them. For example, I write in English to whine without having to really pay attention to what I'm saying. If I did that in my mother tongue I'd get fed up with myself very quickly and that'd mean I'd actually have to do something about my life. So I basically use English to avoid myself.
French, on the other hand, isn't nearly as easy as English is because I haven't studied long enough (and because no one seems to speak French in this country). For some reason, the things I write in French are somehow more real to me than those I write in English. It might be because I actively have to think of what I want to say, which I don't really have to do in English anymore.
I like French because it isn't easy, at the moment but I know I probably will master it in some time.

On the other hand, I don't think it's possible to ever truly master a foreign language.
I mean, I spent about four hours last night to read an English book that has 241 pages. It takes me two weeks to read such a thing in French (because it's more difficult and I get fed up with difficult things very easily). It'd take me exactly the same four hours to read the same book in my mother tongue and I'd understand it just as well.
However, I continue to feel incompetent in English. I don't get ahold of the phrases very easily. I sometimes have to check some words from the dictionary even if I usually can guess what they mean. I don't have the same kind of intuition I have with my mother tongue. I don't instictively know what's the right way of saying some things. Of course, if I lived in an English-speaking ountry I would probably learn to do it better and I would also learn what kind of language is considered appropriate in different situations.
But somehow I don't think I would ever be able to express myself as vividly as I can with the language I learnt to speak first. It's interesting because I learnt to speak English when I was approximately five years old (well, I never claimed it was good English :) ).

What bothers me are the visions of a world-wide language that'd be spoken everywhere or which everyone would master. I have nothing against something like Esperanto and the likes and I wouldn't object to that there'd be a language universally used for communicating (like English is often nowadays). I am, however, devastated when I interact with people who think everyone should start speaking English, for example. I know people who consider the language I primarily use for communicating as unnecessary and vote for starting to talk in English.

I can't think in English the way I can think in Finnish. English words don't express different things to me in every situation, I don't get the hints that natural English speakers can use. I can't make connections with emotions and colours and words in English as easily as I can do the same in Finnish.
And more importantly, I love learning to speak new languages. They're like math problem I have to decipher and I truly find it satisfying to learn even one new word in deffierent language. I learnt how to say 'in self defense' in French the other day and it made me happy. Now, most people I know don't have the same passion with language (you know, I'm fairly certain that 'with' over there is wrong but I cannot replace it with a better construction) as I do and that makes me wonder. If they don't passionately want to study one language to master it then why on earth would they be willing to change their mother tongue?

Ugh. *shudder*

I am fluent in English but I can't do math in English, you know. There's no way I could give up on my mother tongue. Even if the official language of the country/world was English (or something else) I wouldn't start thinking completely in that language. And if I wouldn't do that, perhaps not so many others would either.