1. If you ask us for a payrise, we have to weigh the cost of the payrise vs the risk you WILL leave to get one somewhere else. We KNOW that if we turn you down, there's a chance you're going to be looking elsewhere.
2. By telling us you're dipping your foot, you've essentially given your notice. You just haven't told us when you're leaving. As of right now, you're not going to get the same level of attention and I'm looking for your replacement.
2b. If you don't tell us you're dipping your foot in the water, but we can't give you a pay rise, you'll get more attention and you'll get anything else we can possibly give you. We might give you a nicer desk, or replace your PC before anyone else's, or take you out for coffee more often.