What are the consequences of having an electrical neuromuscular junction, there must be reasons why the nmj is chemical?

Do you mean why are muscular contractions mediated by calcium?

I don't think there's really any reason an electrical synapse couldn't work... it could still trigger the same muscle cell depolarisation, calcium influx through voltage gated channels, and then muscle contraction as the ACh chemical synapse. It would occur faster, which might seem to give it an advantage over chemical transmission. But I think the chemical nature of the nmj synapse allows greater plasticity - the quantity of synapses from a single neuron can change more easily if it's not needed to join directly with the muscle cell - so the nervous system can grow to meet the demands of the muscles as they grow. This is just an idea, but it's about the only advantage of chemical synapses in this case that I could think of. Another major difference that might contribute, though I'm not sure how, is that electrical synapses allow current to travel both ways - the action of the muscle could perhaps trigger an action potential in the neuron going back to the cell body... whereas the chemical synapse is a one way thing.

As billyaxon said, electrical conduction would occur much faster than chemical, however most electrical conduction is intracellular(i.e. not in a synapse). This is what generates the action potential.

I think the main reason that most nmj are chemical is that so that a number of neurotransmitter substances can be released. For example lets take the heart, during exercise more Noradrenaline is released by the sympathetic nerve which innervates the sino-atrial node. This causes the heart to pump faster and harder.

However during rest (i.e. when sleeping) the heart does not need to pump as fast, therefore the parasympathetic innervation of the heart is initiated, which means acetlycholine is released and the nmj; causing the heart to beat slower.

Furthermore drugs can also affect the way the neurotransmitters behave at the nmj. If the nmj was purely electrical then it would not be possible to control different physiological conditions around the body.