There are many, many reasons why a single-pojnt security solution is far better than a solution based on client-side software. Here's a very quick, and likely incomplete brain dump:
1. Personal firewalls can be deactivated by the user --- so why bother at all? Constant and incomprehensible pop-ups in NIS tempt users to turn them off -- trust me on this.
2. Security policies cannot be uniformly enforced, as local configuration settings can vary from system to system.
3. Status reporting and break-in attempts are not centrally logged.
4. Client software needs constant updating, and will either be neglected outright, or consume IT staff time to ensure it is done.
5. Rampant false alarms...
If you have a LAN, you protect the whole thing with a gateway/firewall. You wouldn't secure your house by leaving the front door wide open, and locking individual bedrooms with those cheapo little locks and flimsy doors --- you put a big deadbolt on the entryway, and control who gets a key....
Dan G.