The NIC speed on the Intel platforms depend on the specific board layouts.
Some are Gbit, some are 100Mbit.
Assuming you have an Intel board with a 1Gbit shared network connection the total bandwidth of the connection (BMC and Mother Board OS ) can go as high as 1 Gbit.
In theory you could see the mother board or the BMC connection slow as the load is shared because the other is using the bandwidth, but in practice even when doing KVM redirection the BMC runs fine on 100Mbit.
Using the dedicated NIC eleminates this potential speed loss by running the BMC on its own NIC while adding the securtity of a more limited accecable management network.