Leon, when you are in open water it is easy enough to put the boat in gear, see which way it turns and correct heading on the fly. It’s mainly when you are starting out from a mooring (especially with a single screw vessel) that it becomes an issue, and especially in tight quarters. If I am starting out slowly from a side tie, having pushed the bow out, and want to clear the vessel still tied ahead of me, I want to make sure the rudder is straight before the rudder position becomes evident from my forward motion. By then it may be too late to correct without an “embarrassing” encounter.
When I come into a dock I try to make a habit of centering the helm. And then just before leaving I try to remember to check it again. You can do this of course without a rudder position indicator by rotating the wheel in one direction to lock and then turning it half the number of “lock-to-lock” turns back the other way (on my boat that is 2 and 5/8 turns). A rudder position indicator would help if you have forgotten to do that or have lost track while underway slowly and need to know quickly.
For example, coming out of the Ballard Locks near Seattle it was my turn to cast off. In the confusion I had forgotten to center the helm. There are currents and I found myself dangerously close to a concrete wall on my starboard side that I was moving parallel to but heading toward. Too much left rudder and the stern hits. But if the rudder is even slightly to the right the bow hits. If I apply more power, what will be the effect? Thank God (and Ranger) for bow and stern thrusters! Likewise, I was recently backing out of long finger piers in Victoria Harbor with boats on both sides and little clearance. The rudder has an effect while backing, but it takes some time while underway to know which direction that is. There too, I wanted to know for sure the rudder was centered while making my corrections with bow and stern thrusters.
Bottom line, I can do without it by remembering to center the helm but a rudder position indicator would be nice to have.
John B