Plenty of people have brushed off Brotherhood and Revelations as "fillers" I myself thought of them as fillers until I actually played them.
There are significant plot progressions at the end of each that give you a much bigger perspective compared to just playing I, II, and III Plus going through the complete journey of Ezio and even watching Assassin's Creed: Embers is recommended if you are into the story of Assassin's Creed as a whole.
They aren't so important that you won't get what's going on, but I highly recommend at least playing through the main story missions.
From what I remember the main story missions of Brotherhood and Revelations were pretty short compared with AC2. You could probably rip through them both pretty quick if you skipped the side content.
Or just YouTube the ending for Revelations and get caught up on most of the...uhhhh... revelations.
Yeah, if you skip most of the side content, brotherhood and revelations are REALLY short.
Doing some of it is recommended though, in particular the Da Vinci inventions missions and the Romulus lairs for brotherhood. And the Assassin recruit missions for Revelations, Desmond's story can be skipped, but i'd still recommend it if you don't have a major problem of the gameplay of it. The Lost archive DLC (for Revelations) is recommended regardless, The Da Vinci disappereance is also recommended for brotherhood.
Embers is also awesome.
If you really want to skip right to this though, you'll need to find some good summaries and stuff. In particular you want to know who Ezio was (properly, who he was through Brotherhood and Revelations), and know all the detail about the current time events.
I'm tempted to make an attempted sticky that summaries up the plot of all the games with the appropriate detail really, since there's starting to be quite a bit, and it's hard to give adequate answers to what would be sufficient knowledge to attain without playing the games, without stating excactly what.
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