If you are the IT director of a big corporation, you have no option but to MITM SSL traffic. The alternative is providing a perfect way for malicious insiders to steal corporate secrets (like a whole pile of credit card numbers or the blueprints/source code for the companies latest products). And providing a vector for malware or attacks to bypass all the edge-level intrusion detection systems.
And providing a way for the people on the inside to access things that they shouldn't (whether its pornography, pirated content, or anything else). That last one is even more important in, say, a school or educational environment or library than in a corporate network.