If four members of the group are physical, in the case with a monk being DPS, then it would possibly be the two tanks, a monk, and a samurai, or instead a ninja. Here's the problem. Monk applying 5% physical dps ever 75 seconds to the NIN and two tanks won't beat out the Samurai. Tanks no longer do proper damage. Right now, the only reason tanks have any decent parses is because people keep stubbornly using slaying accessories from 3.4. It won't fly the more difficult the content becomes. Ninja's trick attack may make up for Samurai with the Monk there. The problem? That means there's no reason to have a Monk. Monk's own buff does not apply to it. Only surrounding physical attackers. If you have 2 casters in your party a monk is no longer necessary. A samurai can piggyback off of a Red Mage's embolden and a Ninja's trick attack far better than them. Monk giving 5% to a ninja and two tanks can't even compare.
As it stands, most of the group utility in added this patch is biased towards physical. Most dragoons will never use their dragon's sight on anyone but a tank or a melee since it has pathetic range. Embolden only buffs the physical attacks of anyone not the red mage. Brotherhood only buffs the physical attacks of anyone not the monk. The difference is that SAM does not need utility in order to shine, nor does it need support, where as DRG, MCH, and arguably MNK do. It also works together with all these buffs far better than most other classes, since obviously buffing the highest damage dealer is the best option. Right now, most utility exists to further push Samurai's damage, people would rather sacrifice a small amount of utility to bring a Samurai in to piggyback off the rest. It is not that the other classes don't do the same DPS, but that in certain comps at certain skill levels, there comes a point of "why not just take a samurai?" If a group were to have three casters, the only dps that could seriously compete for a spot against Samurai is a Ninja. That was my original point.
Edit: If I had an "ultimate point," or a tl;dr, it's just that most of the utility aside from Ninja just isn't strong enough to compete with outclassing a Samurai's damage. I've yet to see any mathematical proof that one class replacing a Samurai spot will bring overall more (raid) dps than a Samurai taking that spot itself.