Originally Posted by
Marrilaife
Well let's take example, you're a Rogue and you get a sephuz as outlaw and insignia of ravenhold (another shared one) as assassination, is there a point switching to sub? Not really. How many leggos you have usable for sub? 2, both of them. So if you switch to sub how much BLP you need to get a legendary? As much as 3rd legendary requires (no increased chance, it's only increased for 1st and 2nd). So now on each spec you need the same amount of BLP to get a legendary. So your best bet is to switch to spec you care the most (has best legendaries in the pool or you want to play it the most).
Your BLP doesn't count how many you dropped as which loot spec, it counts how many you own that belong to that spec (and shared ones can belong to all specs of a class, for example - warrior bracers, or to 2 out of 3 specs, for example - warrior execute ring). So for example if you loot spec arms and get bracers and execute ring, your fury spec requires as much blp as 3rd legendary to drop the next one, but your prot spec requires only as much as the 2nd legendary (however if you only care about your dps spec there might be little value in getting a prot specific legendary for example thunder clap belt, however if you tank as well it's a prot legendary you'll get faster than if you wait until your dps spec drops another shared - let's say heroic leap shoulder).
Blizzard explained it once with an example of mistweaver monk, whoever is the Blizz guy with the cinder cat icon.
Obviously there is RNG element to it, you can be super lucky and get legendary "out of line" (one of my alts got 2 of them in the same EN run even though I already had legendary before so theoretically my chance wasn't super high) or you can be super unlucky (my monk and my dk were levelled together monk has 5 legendaries dk has 3...)
But in general it counts how many you own for a spec: if less than 2, you have increased chance, if none, you have massively increased chance. 2 or more - it's all the same. There were several hotfixes to the system around 7.1.5 patch.