Hi,
every component dealing with data must serialize data that needs to persits. it's a must, otherwise you would loose everything everytime you close Unity...
Since PlayMaker has a very specific approach, it can't serialize the simple variables ( floats, ints, string), it's just not enough, it has to serialize its own complex classes because a int within PlayMaker is a lot more than just an int, it has a name, and options like bneing shown on the inspector, or flaged as synchronizable for network, and so this falls out of what Unity support by default, and her comes the trouble that you must serialize everything yourself at that point.
Bye,
Jean