Python multiprocessing tool vs Py(Spark)
True, Spark does have the limitations you have mentioned, that is you are bounded in the functional spark world (spark mllib, dataframes etc). However, what it provides vs other multiprocessing tools/libraries is the automatic distribution, partition and rescaling of parallel tasks. Scaling and scheduling spark code becomes an easier task than having to program your custom multiprocessing code to respond to larger amounts of data + computations.
A newbie question, as I get increasingly confused with pyspark. I want to scale an existing python data preprocessing and data analysis pipeline. I realize if I partition my data with pyspark, I can't treat each partition as a standalone pandas data frame anymore, and need to learn to manipulate with pyspark.sql row/column functions, and change a lot of existing code, plus I am bound to spark mllib libraries and can't take full advantage of more mature scikit-learn package. Then why would I ever need to use Spark if I can use multiprocessing tools for cluster computing and parallelize tasks on existing dataframe?