Seems to me very few people can measure a loco’s speed characteristics well enough to be able to meaningfully set 28 speed steps, let alone 124!
And even if you could, can anyone tell the difference from using low medium and high speed settings?
The primary purpose of custom speed tables are to get two different brands of locomotives to run together as a consist. Other than for that purpose, I have never used a custom speed table.
I don’t use the entire speed table, but I do use CV2, CV5 and CV6 to match different brands of locos and set what I feel is a good max speed. Works really well for me.
I’ve had to use the custom speed table on my two Atlas H-16-44 locos; the built-in Atlas decoder doesn’t support CV5 and CV6, and the loco just runs too fast with the default settings. So for now I’ve built a spreadsheet that extropolates the values out for a switcher, road freight and mainline engine, and use the spreadsheet to set the speed table values. I don’t plan on MUing these two, but I do prefer more “realistic” speed control.
I’ll probably upgrade the Atlas deocoders someday, but I still have 7 or 8 older locos to get decoders for and install them, so it’s not high on my priority list.
I use custom speed tables in all my locos because I want to have fine control over the low end of the loco speed.
So my speed curves jump to a level at step 1 where the loco just crawls, then slowly increases speed up to about step 20 or so, then rapidly curves up to reach top speed.
This gives me very fine control at the low end, and since the top speed on the Siskiyou Line is 25 mph, having lots of control at the low end really helps, since that’s where the crew operates 99% of the time.
I just use the low, medium and high to speed match locos. the 128 is nice but very time consuming and if your DCC unit doesn’t read back what a CV is set to then you have to write them all down or you’ll never remember what is set to what.
***BUT if your interested, I have an Microsoft excel spread sheet made just for that. It is based off the Digitrax decoders, I think I have NCE in there as well. If anyone wants a copy of it I’ll gladly email you a copy of it.
I’m still not convinced but Joe F has almost got me there. 
Seems to me setting the mid-speed CV to a low value would have much the same effect, Joe.
If you have software like Decoder Pro (which is freeware) it is fairly easy to create a speed table, and it can be loaded to the decoder automatically. Decoder Pro also allows an automated load into additional decoders if you have multiple identical units. Low , mid and high do OK, but you can get better matching and finer low speed control with the speed table. The down side is it does take longer to set up.