At worst pure class is always worth it for spellcasters because there are items that allow to cast free lvl6 spell or to recover lvl6 spell. When you can cast 3 lvl6 spells in a day, it's definitely worth your lvl11, and then a feat is often better than a single level in a class.

Worth mentioning for druid lvl6 spells are sunbeam and summon an elemental myrmidon. The heroes feast, heal and vine wall are also worth the spell slot. I guess overcast call lightning would be ridiculous too.