Pros
It's fun and passionate work. Most people feel excited to come to work for the day and it's contagious. The pay in some sectors is very good too.
Cons
They like to pretend you are valued. I worked as a seasonal employee for multiple years on a 7-month term. In the offseason they hire new employees for 5-months so they can avoid paying benifits. The managers do fight to keep their seasonal employees on as full-time so they don't have to keep training people year after year since they leave to land full-time jobs elsewhere, but they never get approved by those at the top. All of this is to avoid paying benefits. Some at the top will admit this too. It's not right to have a revolving door of seasonal employees to avoid beneifts while they claim to "value" the employees.