Hard to say how much good McCarthy did before he turned to causing harm. Yes, we had communists. The vast majority were merely extreme liberals. Some, however, did turn over nuclear weapon secrets to the USSR because they didn't trust the American Powers that Were at the time. Why they thought Uncle Joe Stalin wasn't worse I'll never know, but I don't blame them for having their "what have I done?" moment over nuclear power. Nonetheless, it was treason and trusting it Stalin was an insult to their own reason--I don't know what they were thinking.
And, of course, one politician's "it's a dirty job but someone has to do it" is another politician's "hey, if I can keep this going it will get me reelected", and that's where "Tail Gunner Joe" comes in. Then the focus turned from the halls of weapons research to Hollywood and "the propaganda threat"--and straight off the rails.
I don't know how much was left to be done at the time. If there were more communists in positions where they could truly threaten the U.S. (and I think there were), Tail Gunner Joe was not going to get them. He had already decided that bank robbers were too hard to catch and he could make his quota more easily just hiding behind the billboard and surprising speeders. He was a lousy, corrupt cop, and letting him go on would have solved nothing.