With the parties limited to using TV and internet to get their pre-election message across, preserving the fiction that they still enjoy mass support becomes even more important. Hence the choice of venue shown in the picture. It allows police and party media handlers to carefully control access and it's position far from the Thessaloniki's main streets means that protesters can be kept of sight and earshot.
Also the narrow space (it's the jetty pointing out to sea in Google Maps shot above) means that even a few hundred people can be, with the right angles and lenses be made to look like an adoring crowd. Also since it is outdoors Venizelos can claim (at least on TV) that he is still popular enough to appear in public. What viewers will not see will the thousands of police on duty making sure that Venizelos can appear unmolested outdoors. Claiming to represent the will of the people whilst hiding behind rows of baton wielding riot police is not the carefully honed media message PASOK wants to send to voters on the eve of such an important vote.