A few advantages would be:
-only 3' sequencing so therefore far less sequencing for the same expression quantification. As such the cost of sequencing is reduced, or you can multiplex more samples on the machine.
-less problematic for heavily degraded RNA
Obviously, you won't have alternative splicing/isoform usage, so it depends on what the aim of your research is. If you only care about expression levels, QuantSeq is fine (and definitely better than microarray).