I agree when it comes to the headline enforcement actions (breakups, etc.), which are done in a pretty haphazard way with questionable effectiveness. But I think the existence of the body of law does discourage the most egregious types of monopoly-protection activities. For example, few companies will do things that look too much like outright dumping, stuff like giving your product away for free for 12 months in order to crush your new startup competitor who's secured 12 months of funding.
That example doesn't bother me too much, though there are plenty of smart folks who would disagree. I don't like IE's dominance, which dates back to exactly what you describe (vs Netscape).
But the outcome is that the Internet became a mass phenomenon. I don't think it would obviously have been so if Netscape and MS charged for browsers. Heck, companies used to charge for TCP/IP stacks. Dumping looks pro-consumer in this context.
