Ultimately I see two problems though,
1. sometimes the number doesn't matter or make sense at all. A good example is a macro knob. The value is somewhere between "0" or "1", and synths do let you set it manually (since this is how recorded automation works), but a macro slider doesn't make too much sense IMO.
2. lots of controls deal with logarithmic values. Anything that corresponds to a frequency is going to need finer control when you're tweaking values below 500Hz vs changing a value between 10000Hz and 10500Hz. Knobs mask this pretty well. I'm sure you could build a slider that dealt with this, but a number box would be very weird since you'd want the scroll step to be much smaller at lower values.
Probably not, a lot of musicians develop on the go (planes etc) so they're dealing with built-in trackpads pretty often. You can still scroll but it's not as ergonomic.
I'm racking my brain thinking of what a better interface would be for selecting a number between a range of values, where the number is a point on a continuum and not any specific value, and can't think of one. The equivalent "traditional" UX for webapps would be a slider control, but that's functionally the same and you'd be going against many years of domain-specific common understanding for not much benefit.