Niiice.

Probably very easy to forget. Prophet of Kruphix / Seedborn Muse are big effects that you're not going to forget, but this is subtle enough I can see players completely forgetting it until after it would have mattered.

But I like the idea enough I'd say it's worth playtesting to see if it works.

It would mainly be for control decks or other decks doing things at Instant speed, of course. It lets you end the turn with only one mana open for Mana Leak or what have you, or allows you to tap out and still use your Druidic Satchel's ability.