I found the shield spell pretty useful. My last two mages had very easy sienna locations (one was pick-up) but I rarely went and got them because I just didn't need to. I might have gone through one sienna rod every 2-3 sessions because outside of the longer duration raids, I wasn't fighting that rough of opponents that often that the re-use timer got in the way.

The limited shield spells aren't "on a stick", which is what a lot of mage players want/demand, but it's infinitely better than having zero access to shield if you didn't have a supply of wands, and in many cases, the difference is negligible.