The delinquency letter you send out to deadbeat clients that says you work with XXX debt collection agency to recover fees is likely to get a better response rate than the one that says you'll "pursue all options".

Note also: a good corporate collections agency might know tricks you wouldn't think of to spur repayment. For instance, you can ding people's D&B reports with collections issues (who knew?), which can cause problems for some companies with their other B2B transactions.

Some unrelated case-in-point: I'll pay parking tickets in jurisdictions where they're handled by collections. I won't bother paying parking tickets elsewhere, even when they have hard-and-fast "3 tickets and then we boot you" rules. The collections agency is just a bigger headache than getting a boot off.