The main disadvantage to entering into an employment agreement is the lack of flexibility in the employment relationship. Terms the employer otherwise might change in its discretion become fixed by the employment agreement. You might lose the ability to renegotiate the terms of your employment contract during the term of the contract.
If you decide you need a written employment contract, make sure to review it carefully with a lawyer's help. In some situations, handshake agreements may actually work to the benefit of an employee. Many of the terms that would be part of the contract are documented in paychecks and company employment manuals. These manuals typically outline things such as vacations, paid holidays and sick leave. There are laws designed to make sure employees are paid for overtime and protected from discrimination on the job.
But there are still times when it's critical to get your employment contract in writing, especially if you have a unique job or if you're going out on a limb by accepting a job. In addition to obvious issues like salary and employee benefits, you'll want the contract to cover specifics such as: