SSD as boot drive is certainly nice (machine boots faster, applications open faster, etc).
A conventional HD is fast enough to sustain 100 solid/contiguous 24Bit 44.1/48k tracks of audio.
Thus, for many folks, SSD is overkill for an "Audio" drive.
If you're working at high sample-rates, especially with dense projects, SSD is a good solution.
Disk-streaming sample-libraries are where SSD really shines.
A fast SATA-III SSD is about 3x the speed of a conventional HD (~520MB/Sec).
That translates to 3x the disk-streaming polyphony from sample libraries.
If you're running a sample library like EWSO (which only allows a single drive location)... and you need massive polyphony, a cost effective solution is to put a pair of SSDs in RAID (sustains ~1000MB/Sec).
If your disk-streaming polyphony needs are crazy, you've also got the option of PCIe x4 (or m.2 Ultra PCIe x4) SSDs.
These drives sustain 2500-2600MB/Sec.
In short, let your needs define the drives used...
Choose them based on performance and space needed to accomplish your goals.