If indeed it did not relieve the symptoms or slow the progression, it would have no point. But, I think it can have a role to play in these circumstances:
- where you don't perceive the difference in your symptoms;
- it works with other drugs to take you across the therapeutic threshold, and keep you there in a more stable way.
For instance, my drug regimen is (12 years post diagnosis):
5x75mg Stalevo, levodopa equivalent dose = 500mg
8mg ropinirole CR, LED=160mg
1mg rasagiline, LED=100mg
Daily total LED = 760mg
The Stalevo is relatively fast acting, but the other two provide more stable foundations.