Real World Details
Developed in the 1960's the BMP(Battle vehicle of the infantry) set the standard for protection and versatility. While other countries possessed ACP's, the BMP-1 was the first IFV and enough firepower to be able to stand up to tanks. With its cannon and ATGM's the BMP-1 set the standard for all IFV in the future, followed by BMP-2, trading its cannon for an autocannon, as by the 1980's a 73mm gun was tough to be less versatile than an autocannon.
In Game Details
Over the BMP-1P the changes are:
-Same cost (25 points).
-Improved armor +1 front, rear, and top. (2/1/1/1)
-Reduced speed by 5km/h (55km/h).
-ATGM replaced with Grenade launcher - versatile weapon that can shoot even at helicopters, but only at very short range.
The BMP-1D differs from the other BMP-1 variants in that it does not have an ATGM system. In place of it it has a grenade launcher. With this setup the BMP-1D is more of a infantry mopping and close quarters assault vehicle.
On top of the weapons change, for no increase in price the BMP-1D has +1 armor all round over the predecessors. Meaning it can survive a frontal hit from some NATO ATGM's and some tank shells. This extra armor comes at the price of reduced speed by 5km/h, but this is only minor.
Since the patch that now allowes the BMP-1D to fire both weapon systems simouteniously it makes the BMP-1D an interesing combat vehicle.
The BMP-1D's grenade launcher can be used to stun some poorly armored vehicles, giving it that extra second to get a shot off on its main gun that can often mean the difference between life or death.
The BMP-1D's are good in supporting infantry attacking forests and villages as the grenade launcher will quickly stunn and panic the enemy infantry, giving your troops the edge they need. However this is problematic because the BMP-1D carries only 45 rounds for the grenade launcher, meaning that after each attack it requires resupply, as its the grenade launcher that makes the BMP-1D stand out over the other BMP's.
Because of its improved anti-infantry abilities it can often prevail with its troops over superior infantry in forests and buildings(flame tank will still do a better job in this capacity tho). And don't forget that its main gun, can still do damage to tanks, and even Leo 2's from the side.