In Creative, when you have a full hotbar, block-picking doesn't work with a tool (axe, pickaxe, shovel, hoe, flint and steel, fishing rod, shears), weapon (sword, bow), or piece of armor in your hand. This issue is covered by MC-327, and is resolved as working as intended; the reason given is to prevent players from accidentally replacing a customized tool with a block.
However, enchanting such an item actually removes this immunity.
(Furthermore, there's nothing similar in place to prevent the destruction of firework stars, maps, enchanted books, written books, or non-tool items with custom NBT data, much of which is harder to reproduce if lost, especially without commands.)