Biological advantage of electric synapses
Having just read this article (http://www.ncbi.nlm.nih.gov/books/NBK11164/) there are several advantages/functional reasons that seem apparent and important in having electrical synapses with gap junctions compared to just a very long neuron.

a) signals in electrical synapse can be bidirectional.
b) electrical synapse synchronize electrical activity among populations of neurons.
c) The large pores in gap junctions allows passage of ATP and other important intracellular metabolites, such as second messengers between neurons.

I'm sure there are plenty of other reasons but these should provide a better picture in terms of the functional significance of having electric synapses.

Hope this helps!