[Eomer of the Rohirrim]

I agree. And not only that, but in a roundabout way it also detracts from Merry to have him at the Black Gate.

No-one really remembers Merry's part in the death of the Witch-King. The evil guy dies and, before you know it, the two hobbits are off with Aragorn again. Having a couple of extra seconds showing Pippin alone without Merry would have underlined the sacrifice Merry made, as well as giving Pippin some much needed centre-stage (along with the Troll-slaying, of course).

As it is, it just seems like the two comedy-hobbits are along as baggage; and they are instantly forgettable in the scene at the Black gate, at least to the audience who just think they're for amusement..