I'm sure Joris will understand the idea from the subject, but I'll elaborate...
While playing back content with audio I have had dancers choreographed to the music I'm playing back, singers singing to the music I'm playing back and live instruments playing along the the audio I'm playing back. in all of these situations the last thing you want is any variation in the speed of the audio play back. it is quite easy to miss-click the global speed control and not realise you have changed the speed.
The most intuitive option would be another transport type like BPM, Timeline or Autonomous but I suspect the code might be fairly tricky hence the idea of just being able to disable speed control such as the 'Disable global effects & blend modes' toggle.
Obviously this isn't a A1 priority type feature it's just a little safeguard against human error, I have stuffed up this twice, once when I set up a show synced with abelton without realising global speed control was actually set fast. So when I right clicked to make sure it was set to zero... you can Imagine . The other time when dancers were synced to the playback and the promoter asked me to pull back the brightness, and I missed the master fade with my click...