Is there any pitfall to using resistors in series AND parallel for an LED array?
Your LEDs may or may not work exactly as you expect, depending on what you expect.

The change in brightness when operating the switch will vary depending on whether you have 1 or 9 LEDs lit. With 1 LED, the change will be small, with 9 LEDs, the change will be large. 

With the switch closed, any number of LEDs will have the same brightness. With the switch open, as you turn on more LEDs, those already on will get less bright.

If you eliminate R8, and PWM the LEDs, you can achieve a constant brightness change regardless of the number of LEDs lit.