Having a cluster of antennae close together at the back of the rack is a bad idea, and not just because it looks messy.
Each antenna, although fitted to a receiver, also functions as a transmitter, albeit at a very low power level. (It's the nature of receiver electronics that a small amount of signal is sent back to the antenna.) This means that each receiver is surrounded by a cluster of additional transmitters which is bad news for RF performance.
Also, as frequently demonstrated on this forum, antennae being located at the back of the receivers is rarely the optimum position for them as good line of sign is generally limited and proximity to other equipment is likely. Getting them up in the air and away from other equipment is always beneficial, even if just using the included antennae rather than the P180s.