Sonic Pi has three broad goals. One is to give coders a very low-friction entry point to live coding music. Second is to create a musical instrument capable of being taught in school music lessons. The third (and primary) goal is to provide a way to engage learners with coding (of any age but typically in schools).

The numbers primarily work with goals 1 and 3 because you don't feel like you have to know any music theory to make noise and simple melodies. They also work with 2 because it's pretty easy to transpose (+ 12) and modulate the main notes. Using floats also enables you to specify notes between semitones (play 30.235).

Additionally...

Sonic Pi also supports specifying notes as symbols: play :Eb3. You have access to a large corpus of scales and chords and you can also modify the BPM globally or local to specific threads. You can even access scales using positions such as :iv.

We're always looking for new ideas - especially ones that provide new way to manipulate music concepts through code that give people greater access to concepts they would typically find hard to understand through formal theory.

Also, we're just looking for ways to make Sonic Pi more fun to jam with...