Power rune specifically states "...You don’t need to BIND magic to use this spell." indicating that you can use the same rune more than once in this particular case. No other spell specifies this exception to binding. Or at least that's how I interpreted it; it becomes a rechargeable rune after the initial cast. I'm happy to admit i was wrong if that's the case though lol, learning is good.
The spell creates a magic item (without the use of the spell Bind Magic). If you create a power rune with 2 WP, you have a power rune with 2 WP. Those 2 WP last until you use them. That particular symbol (carved or drawn) can't be used for spellcasting again. When you have used those 2 WP, you have a symbol that you can no longer use, but you can draw another, or if it is carved, use the material to carve another symbol and then cast the spell again.
But it is your game so you can do as you wish of course.