Fees for baggage above your allowance can entirely negate any savings from cheap airline tickets, and Jetstar is one of the worst offenders. However, if you happen to be flying this Friday, you don't need to worry — Jetstar staff will not be processing excess baggage fees as part of a protest over their salary and conditions.
ABC News reports that the 'No Weigh Day' protests will be taking place at Sydney, Melbourne, Cairns, Brisbane and Gold Coast airports. As protest tactics go, this seems a clever idea — it will cost Jetstar money ($50,000 a day according to union representatives), but passengers won't be inconvenienced. Note that Jetstar staff will still be weighing bags, and I'd assume that bags way over the limit might still be rejected on OH&S grounds.
Jetstar strikers announce 'No Weigh Day' [ABC News]