Control is a hard thing to give up, and after you have made your record you now need to put it in someone's hands to do crucial final tweaks. If botched it can ruin the experience of your record or if done properly it can accentuate and give your record a breath of fresh air. Mastering is the last creative step in making any record that has the potential to be great get to that greatness. Unfortunately, many times trying to keep control at this stage can be one of the things that brings down your record. The relationship between an engineer's speakers and their ears has a value which can never be calculated. That said, many times artists/producers will walk into a mastering engineers room and try to make critical decisions on the sound of a record when they do not know the room very well and can't make proper judgments. In the hundreds of records I have made over the years, one of the smartest decisions I have learned to make is to stay home from the mastering session and have the mastering engineer send me files to listen to on my own speakers in my own room so I can make educated decisions on how I want the record to sound. By being in my own comfortable environment this has made it so I am happier with my records and know I am always making the right decisions.
To the contrary, certain mastering houses can ruin a records dynamic range if you leave it up to them. I've even had records sound worse when I've got them back. I prefer to be there. It's my record and my sound, and therefore I have no one else to blame if it goes south, since I was there the whole time.