The thing is you're comparing not doing anything to telling her you like her. But there's a third alternative that is "showing" her you like her. Honestly every time I've doubted in kissing a girl, it's ended up that I should have. Sometimes things work out anyway, but other times neither of us did anything, things fizzled, and I found out too late that she liked me. Certainly saying something would have worked in many of these cases. It's better than not doing anything, but it's not the optimal solution.
When you *show* a girl that you like her through physical means, it's a lot more powerful because:
a) You show you got balls
b) She's understands that you like her, but she's still unsure of how much or what kind of relationship you want. That's exciting for her and it'll make her spend a lot of time thinking about you.
c) Spoken word can never match physical contact in terms of provoking emotion.
Just yesterday I made a mistake, which nevertheless was a lesson. A girl and I have been flirting back and forth. I wasn't too sure of her interest, but we play with each other, arguing and then being sweet. I left the city where we were yesterday afternoon and when we said bye she said "are we really going to be able to hang out being in two different cities?". Which really means "can we have some sort of relationship being in two different cities?". When I left, I touched her face. I had done kino before but I had never touched her face. You wouldn't believe the look on her face. It was like if I have put my **** in her (in a good way). I should've kissed her but pussied out cus we were at work. Anyway my point is that that simple touch was so much more powerful than anything I could have said.
Now, before I ramble too much more, there's another thing about expressing feelings that concerns doing so when you already have a relationship with a girl. My experience is that every single time I've gone all out in telling her how much i like her, interest level has plumeted. You'll see similar experiences on this board.
See what happens you express such liking is that you end the game between you two. Suddenly she realizes--consciously or subconsciously--that you're hers and that she doesn't have to do anything more to win you over. But if you keep you mouth shut, while still showing physical interest, she can never be sure exactly what you feel for her, how long you want to be with her, what your plans are, etc... The game continues, and whatever she might say, that's what she wants.