I'll try to answer some of your objections.
(1) You're right that "sample of 1" is an expression found in experimental science. But how do we know that a small sample is no sufficient reason to accept a hypothese if not by using logic?
Also: Read PG's essay once more and tell me: What reasons did he present to support his hypotheses, other than merely: "I didn't understand philosophy, therefore all current philosophy is useless"? He's basically making an appeal to authority, namely his own authority. Hardly a good reason.
2) PG asserts that early philosophy was inspired by maths. But was it, really? How does PG know? In fact, both fields probably inspired each other. This is one of the examples that makes some philosophy useful -- examples that PG simply ignored.
3) Just because you agree with PG (or had a similar experience) doesn't make any of you right. However, I agree that philosophy has had a quality problem every now and then. But that doesn't render all philosophy (or its method) useless.
Also: I'm wasn't trying to say: "I think he's wrong". I say: "Where are his arguments to believe his assertions that he can find a quick way?" Even the more practical sciences have had some really strange ideas when looking at them in hindsight. If usefulness is such a good guide, why did they run into these errors?
4) Indeed, all kind of thinking produces more thinking. Homo sapiens is about 40.000 years old. Then, what did they do the first 37.000 years if mere thinking is sufficient to find the meaningful and true statements?
Finally, attacking the presentation is often the same thing as dismissing the argument. For what are his arguments? PG can point to some bad philosophy, granted. Everybody can do this for all kind of fields. I can point at some really bad software. Is therefore all software engineering useless?
PG wants us to believe his assertions that the philosophical method is inherently flawed. But he presents no arguments other than his own experience, some selected examples and that's about it.
If you want to know what's useful about philosophy, look no further than Socrates: His way to ask people critical questions about the meaning of words, the reasons why they do have the believes they have, and his way to find the differences between good and bad reasons (namely by debate) still is what philosophy is all about. We need the bad thinking to find out why it's bad. Philosophy proceeds by discussing and recording these issues.
You might call it as well critical thinking.
Our small discussion here hopefully shows why this is useful. Trying to dismiss arguments makes you a better thinker.