there any reason to keep the speedometre in this day and age of GPS?
As other posters say, Speed over water is very different than SOG and useful to know. What many people don't realize is that the GPS speed is an average over a longer period of time and not an instantaneous velocity. Thus, you could be slowing and not know it for a minute or two. SOW is an important piece of information when determining the effect of sail trim.
Example - last summer, we eased the traveler 2" and saw an immediate 0.5 kt increase in boat speed. With a GPS, the effect would have been averaged out and difficult to determine when factored with other variables.
I don't look at GPS speed when determining performance.