Is spark widely used in bioinformatics?
The only bioinformatics tool other than GATK4 that I am aware of that uses spark is Hail (a Spark based replacement for Plink). Hail is also supported by researchers at the Broad.

Most places I have worked at have not switched over to Spark.  As such, I don't think it is widely used generally. Therefore, I don't think knowing Spark qualifies as a necessary skill to learn for bioinformatics at this time.

However, I think Spark is a superior paradigm for handling the large amount of data that bioinformaticians routinely deal with, and I think we will soon see the field move towards using it more.  Any bioinformatician would do well to acquaint themselves with Spark and to play around with Hail.