The biggest benefit of Improve Shield Wall imo is when it locks in that kill when healers are literally sitting on 0 mana. I think we all know how devastating wiping on a boss when he's at 1 or 2 percent can be. It can literally halt your guilds progression that week to a standstill. The positive re-enforcement of downing a boss (not to mention the bettering of your gear with loot) is a massive boost to your self and your guild. So if this skill only wins you one progression fight ever instead of wiping at 1% then it was worth it.