If an accounting internship is optional, there are many factors to consider. The students who have been leaders in their college accounting organizations and have been courted by several CPA firms will likely have job offers without participating in the internship program. Other students with fewer things going for them and who are burnt out from studying accounting theory, may need to actually work in the accounting field before taking another course. For these students an internship is an important opportunity.
When I had advised students on internships, I asked whether they will be graduating in May or in December. The reason for my question involves the firms' busy season which is usually January to April. I believe that an internship that results in a December graduation date is advantageous, while an internship that results in a May graduation date could be a disadvantage. Having an internship that results in a December graduation date is likely to mean having two busy seasons of experience within six months of graduation. This can be beneficial if the CPA firm looks at the number of busy seasons of experience when assigning additional responsibilities, promoting, etc.
As the coordinator of an accounting internship program I often read former interns' comments such as "I wasn't sure if accounting was for me. After this internship I am now confident that I will be a successful accountant. I also have a new enthusiasm for my remaining classes." The internship also allowed many students to demonstrate that they were much better than their GPA indicated.
In short, some students will excel in their careers without an accounting internship. Other students will excel in their careers because of their accounting internship.