The strumhollow is certainly not redundant, in fact dozens of thousands of players pick & strum in that area. That area provides a slightly stronger and brighter tone which is precisely what many dulcimer players want.
A huge majority of dulcimers are built with strumhollows & that is driven by public demand. If nobody wanted strumhollows then builders would not build strumhollows.
Hello Corvus. The best I understand it is that the stronger and brighter tone noticeable over the strumhollow is due to picking/strumming near the end of the string length, not the hollow itself. An easy demonstration of this is if you strum the open string all the way at the top just above the first fret, the tone is nearly identical to over the hollow. This leads me to believe that the same tone could be accomplished without the hollow. It's also worth noting that the demand for strumhollows does not imply an actual utility function to them. A couple builders have already mentioned that they don't consider it functionally important, but know that their customers want it. It is aesthetically nice, and more importantly it is the norm, but is it actually useful?