In my experience it can be good or bad. But, mostly bad.
There are many famous consulting/contracting companies which provide contractors for testing, for short term or long term. Often, the contracting company’s goal is not aligned with the client. There is a often a tendency to increase the head count and billable hours. This leads to more communications (since more people) & hence more chaos, and unnecessary fragmentation of work (need to keep the extras busy). If you have too many of these extras, then your under occupied, full time employees will start looking for new employers.
To make outsourced testing work, ensure that you have a competent, full-time, well-compensated & happy technical team. This team can clearly set the expectations of the contracted testers, guide them and check if the requirements have been met. They can ensure that the contractors do their job and do not deceive the company. Make sure that the team interviews the contractors because the hiring standards of the contracting company might not meet the client’s needs. Without such a team, the client could be in trouble.