I've always thought it would be much more simple if the user was able to just type it in. The idea you've brought up is similar to that. It would be cool because not only will it make changing speed mods easier, but when people are training to complete a certain song using the rate option, they don't have to go to a text file and do make a whole list of calculations to make sure their favourite speed mod equals rate. XD
Typing in the speed mod would violate the notion of only using mapped keys, and only be an option for keyboard players. This method will be usable by people playing on pad too.
Mmods and Cmods are going to adjust for the music rate, so the player won't have to worry about it when using an Mmod or Cmod.
Side note: The increment sizes are adjustable through a pair of option rows in ScreenOptionsAdvanced. In this screenshot, setting "Speed Increment" to 10 and "Speed Multiple" to 10 means that the speed mod will adjustable in increments of 10 or 100 (or 0.1 and 1 for Xmods).
< maxvg1> shakesoda: then why do i still play lol
<@shakesoda> because you're an ITG player. And thus, a masochist
<@shakesoda> Kyzentun: I think you might need to put down the meshes for a bit