My 2 cents - yes, it lacks a few things but for what is supposed to do, it is very good for setting up out of the box but if you want to customize it then of course, you need to know what you are doing or need to get someone who knows how to do it or buy plugins. It is good and fast for what it is supposed to do and it takes a lot of work to put out and keep this type of product and Osclass team does not get enough credit for their work.
It's built to generate plugin sells. That is why many functionalities are made as a plugin and not built in the core.
I am glad that many of the things are not built in the core (such as a messaging system that Timmy is looking for) because just imagine, if most of these are built in to the core product how big, slow etc., the code will be rest of the users. Every user wants some plugin or the other feature and when everything is added it will no longer be a efficient classifieds system and a good one at that.
With regards to plugin sales, they have to make the money some way and I think really the money they make from plugin sales is peanuts and I am sure it does not cover their expenses or pay for their living. Everyone has bills to pay - so everything can't be free (except in a sillicon valey model where customer is the product and they can ask the wall street to print the money by the truckload). Also, it allows others to create plugins to earn some money. If the core system has to be free then there must be some way to money or else no business/product can survive - as simple as that.