You could also ask what is the downside of NOT upgrading.
Well sooner or later some browser updates will start to make troubles. What might work today might not work in certain browsers later.
Also if you don't update than at one point you might have to go from 5.0 to 9.0 which could be more difficult then updating it from time to time.
3 years back I updated my sites mainly for new features that my clients were looking for. Now there is more stuff for big players comming then long awaited anhancements.
So my updates are done to keep my sites running as hasslefree as possible.