I agree that for the warning MAYDAY would probably be a bit too much, but as has been pointed out if you call PAN at least we know that you may have a problem. In our environment even the use of the technical problem declaration would be better than not saying anything at all. We on the ground are guided by you, the pilot, in these situations. If you don't declare it, I can't really start to put the safeguards in place SHOULD it go badly wrong. We would rather alert people to a POTENTIAL problem than not say anything. It costs nothing to alert the emergency services and then cancel it, in terms of loss of life, should it go wrong, the penalty for not saying anything is significantly higher in the harsh environment that we work in.
There have been several instances where someone has not said anything about a problem, however minor they feel it is, which has escalated and they have not had time to make the call to us for help. I, believe it or not, as a controller have a vested interest in seeing you back safely, I do not want to have to go home at night having had one of "my" aircraft crash due to a problem I was not aware of. I am not hard enough to say "He didn't tell me something was wrong, so I don't care what happens". Make the call, we don't get angry if it all ends happily......