...So with all the excellent charts and charting devices we have these days is there really any reason to cut another hole in your hull? It seems like a depth sounder is really only good for telling you what you just hit...
First of all...as Joe said you don't need a hole in the hull to install a good depth sensor head. They will shoot a signal through a FRP hull (don't know about wood or steel...coupling with the water column might be more problematic).
Second...because unlike the plotter position you suggest most rely on...a depth system is nearer to real time output and more likely to warn you of impending doom.
Third...only the highly trafficked world traveled by first world nations have excellent charts. Many soundings are a century or two old...and the soundings may be quite accurate but charted positions were very inaccurate until the satellite navigation age. So your position on the physical world may be accurate but the chart may not reflect that physical world but be offset by several miles. There are several stories of such occurrences with both good and bad endings in the coral shoals of the South Pacific.
Fourth...as pointed out charts in plotters and on paper are aged data...not real time and likely to be in error due to intervening storms or just simply because it's impossible to keep all chart info near real time.
Fifth...a good sounding in the absence of other navigation info can sometimes give you a pretty good fix on your position depending on bottom slope and contour.
Sixth...it's one of the lowest cost systems you can get to assist in puttering about in the water...less than a hundred bucks for a reliable no frills system...
...so why wouldn't you have one since they provide such important navigation info?