There are CDR drives, and there are CDR drives. Since we don't require everyone to buy an MTU CDG Certified CDR burner, we have to cover our... bases.
The drives we sell are capable of running at full speed, and writing a good CDG disc when you use MTU Silver CDR Media. 
When you use cheap CDR media, most can't write at the higher speeds!
Also, there are some CDG Karaoke Players that can't read some discs written at higher speeds. That is one reason to drop the write speed down... IF you find you must run on a Player that won't read a faster-written disc.
Furthermore, if a disc is very scratched, you might get a better read running at a slower speed. We haven't confirmed this in quite a while, so don't hold me accountable on this point.