EI does not have much purpose if you already have BB and Shaman, there's few farming places where it can outdo both classes and it's survivability isn't great for sustained farming, the only real place I can think of being the 5 mob screen in Dreadrock as a more common farming spot.
On the topic of GB being worth or not, it's definitely worth getting, even with having LDK. It's DPS is pretty close to LDK and it's survivability is also up there, even though it relies heavily on decreasing the enemy's hit chance for peak performance, hence why it can't (or sruggles heavily) with soloing Blood Titan, while LDK just breezes through it.
GB also has quite a bit more supporting capabilities in it's kit, by being able to reduce damage and hit chance by up to 80% each for the majority of the battle (with correct timing) as well as haste debuffs and an almost loopable 60% resistance debuff, it's way more interactive than LDK, which imo is a pretty big deal.
It's combo reliant nature does come with a couple downsides though, missing a crucial attack can set back your overall performance, but when everything goes in your favor, GB outshines LDK in pretty much every way imaginable.
GB also comes with insanely high crit rate by default as well as heavy crit rate and crit damage buffs, which more or less gives you 100% crit rate at maximum stacks. It's very satisfying to hit 3k+ auto attacks very consistently.
And a little side note about this, on the testing servers (currently), GB has way more potential than LDK, which is currently our canon for post rewrite class balance.
But as Aura Knight said, if you need those extra spaces for something else, there's no need to get either.