Because you only get a rare in seven out of eight packs, on average, you count the rare slot as 7/8 (or 0.875) of a slot
Not only are you playing 5-color good stuff, but youre also playing 5-color expensive stuff
More useful and powerful but so expensive it is awkward to get off even in the best of ramp decks
The most interesting thing here is the ability text, and although its unusual to see this in blue, its just not something thats usually going to be useful
Add two more Merlin, Goat cards and three more Bibbidi Bobbidi Boo cards - up to your decks maximum - and you could be looking at a lore windfall from that combo alone
That means you get to use a larger variety of cards than a deck in a format like Standard, and you also get to pick a Commander to represent you on the battlefield