Old thread, but here's my experience with adding creep speeds. This was on a different Kubota M series model, but the overall situation was about the same. The tractor came with 8 speed transmission (4 high and low). I bought a rear mount Orsi flail style ditch bank mower for use on the machine. When mowing ditches with this mower 1-low was often just a bit too fast, - I was either scrambling to keep up with mower position adjustments or riding the clutch.
After a while I burned out the clutch.
I ordered the creep speed kit and had the dealer install it for me. I wish I had ordered the tractor with it from the start, but didn't anticipate the ditch bank mower issue. With the creep speeds installed, the ditch bank mowing is much easier and I do a much cleaner job as I drop down into creep when necessary.
Anyone who says creep speeds aren't useful probably hasn't done any work where the lower speed options are relevant. If you think you might find creep speeds useful, you almost certainly will.
Here are some photos of an Orsi RB215b on a different tractor.