Well, we seem to agree on some issues: Not all philosophy is bad or useless. Some might be. So, let's say "Some philosophy is useless." PG presents a few quotes and we may conclude, he proved his point.

But did he present any useless philosophy or just badly written philosophy? Or does it just appear to be useless because we don't immediately understand it?

The latter case basically boilds down to an argument from ignorance: "We don't understand some philosophers, therefore they must be useless (or bad or false)." Many people don't understand much of contemporary maths either, but that's hardly a good reason to call it useless (or bad or false). In contrast to maths, though, many people assume to be able to understand it.

The former case is also hardly a good reason to call something useless. Kant, for instance, is bloody complicated to read (at least, for me, and I'm German). However, his arguments still make sense -- one just needs to read the literature that explains Kant's ethics and meta-physics in a more accessible manner. They were also quite useful for other philosophers.

You say you can find bad philosophy more easily than I can find bad maths. Maybe, but since this is basically an empirical question, our small sample won't be sufficient to prove anything. On a side note: It's easy to find useless maths -- in the sense of published proofs that turned out to be wrong. Since they didn't prove anything, why have they been published, at all? For examples, look at the history to prove Fermat's Last Theorem. Quite a lot of them failed or were simply wrong. What a waste of time.

However, your test to find useless philosophy also suffers from the lack of a proper definition of "useless" and "useful". Just because you can't think of any use, doens't mean you're right -- this is just another argument from ignorance.

Let's consider ethics, for instance: It is basically useless concerning empiricial results. However, it's still useful in many contemporary discussions; for instance, animal's rights, abortion, women rights, death penalty, justice, atheism, and many other issues.

Concerning your question whether some other form of argument could possibly exist to support PG's claim? Sure, from an empirical point of view: A proper definition of "useless" and a proper random sample concerning philosophical works. This would at least support the conclusion that, say, 35% of philosophy turned out to be useless.

But PG made two additional arguments:

(1) Any resulting number of useless philosophy is due to the current philosophical method that is motivated by studying the most abstract problems.

(2) A different method that starts from studying practical problems and builds up to abstract problems will result in a smaller number of useless philosophy.

To support (2), an argument from analogy would help: Study a model of the proposed method (thankfully provided by science) and establish the empirical fact that is produced a smaller percentage of useless results.

Of course, this can easily be refuted by attacking the analogy. The methods of empricial science simply doesn't translate well to issues philosophers care about; such as ethics.

To support (1), one would need to start from the premise of the current method and deduce that it will lead to a certain number of useless philosophy and that there is no other possible explanantion. I doubt that it can be done, but I may be wrong.

Just to present a counter-example: to establish the habit of presenting arguments in a more formal way could help to distiguish good and bad philosophy much easier.

However, the point is that PG -- although his intention is apperently to "work through a subject to understand it" -- fails badly. He hardly worked though and he probably still doesn't understand it. Maybe, if he would have studied philosophy more carefully he could have made better arguments for his case.

Which shows how careful one should be about certain words: From a philosophical point of view, namely, PG's work was useless, indeed.