Newport... you hit it right on the head. Most people want GPS for navigation, and ETA, not for speed perse' But knowing SOG is essential for ETA, and travel times. It is just as significant for boating, however, Speed over Water, is essential for all of us as well because it tells us our best trim settings for a given wind (assumes steady wind).
You are now starting to get JUST how complicated sailing can be (about as complicated as you let it)... for those of us (most of us, even the so-called cruisers only) who are after the "perfect trim," it is a life-long unachievable goal. You are catching it now... Don't let it rule you, but don't give up on it either. It'll make you a better sailor. It will make ALL of us better sailors.
Don't get "instrument i-tus" either. Gadgets are cool, but remember they are a modern phenomena. The old-salts sail without them, and can smoke some of the young punks using all the gadgets. A knot-meter, and a depth sounder are pretty essential gadgets though, and a great start.
I am with you though, I JUST have a GPS (actually my phone running navionics)... my goal this winter though is to get a knot and a depth gauge to get me better information on my sailing. Will I still use my GPS (absolutely), but those hard instruments I feel are pretty essential for me going forward.