The reasoning is very simple actually; at 275 they are amoung the cheaper formations and will never be the BTS, they are faster than LR, more resilient*** have MW shots for RA targets and help keep a high activatioin count. They provide excellent 'alpha strike' capabilities to take out enemy threats in the opponents rear etc, and are cheap enough to be to sacrificable if necessary.
Pairs at 500 points are twice as powerfull, but threaten to be the BTS unless the marines take a second, even more expensive formation, which in turn means that the activation count is reduced from and average of ~ 10-12, to an average of ~ 8-10. Being a single formation constrains the number of targets that can be attacked with MW, though it also allows the player to 'swap the leader' retaining their battlefield presence a little longer.
It is the severely constrained activation count that is most concerning, hence my prerference for 0-1 singlton Warhound 275, pair of WHounds 500, which does not constrain the Marine player as much if he needs his "Warhound fix"
***Being Fearless and have shields means that being broken is actually an advantage, allowing them to be moved where they can still threaten when they rally in the next turn, while preventing Hackdown hits etc
