Siri is actually the perfect interface for a clock app. It needs only a limited grammar, IE:

(set|stop) clock (for|at) ([0-9])(AM|PM)

which improves recognition, as opposed to scrolling through wheels to select the right time as you would with a GUI.

Possibly. But even if the clock app was simply a single "start timer" button (or whatever function the GP was using) simply getting to the app can be Slow. Compare "take Out phone, unlock phone, find clock app, open app, do whatever" with just saying "hey Siri set a timer for 5 mins" or "take out phone, press and hold home button, tell Siri to start a timer"

You'll end up with 80 alarms though.