With a merc-ID, you are allowed to hunt everywhere. With an OSC-ID just in Liberty and where OSC has a base. Its the same for all corporate IDs. So mercs arent unnecessary.
How many corporate-IDs players are out there and hunt actively pirates and their economic adversaries?
And how many mercs, bounty hunters and freelancers do you see?
So it is not the problem that there are so few mercs. And this little fix wouldnt change it.
A system actively cleared from any pirates is better than a system full of them just waiting for a Liner without escort. Because OSC has bases that are so distant to any immediately help, OSC has to be some kind of police for these systems. Not only for OSC-vessels. Also for Independent traders and other people resupplying their stations.
Edit:
And with a corporate-ID, you arent allowed to fly a capital ship. Show me somebody who would change his battleship for an OSC-ID'd civilian bomber?