I recently had a search engine hit on the string, "hume thinks metaphysics is crap," and so I can't resist the obvious response, which is that he explicitly defends metaphysics in Section One of the Enquiry Concerning Human Understanding. There he divides philosophy into two kinds: the easy and humane, and the abstruse and difficult, commonly called metaphysics. Aristotle would be an example of an abstruse and difficult philosopher, Cicero of an easy and humane; Locke's abstruse and difficult, Addison's easy and humane; and so forth. Hume notes the easy and humane approach to philosophy has a number of advantages: "It enters more into common life; moulds the heart and affections; and, by touching those principles which actuate men, reforms their conduct, and brings them nearer to that model of perfection which it describes." He insists, however, that we need both, and proceeds to defend metaphysics from those who wish to do away with it altogether. A rough summary of his arguments:
(1) The easy and humane approach philosophy needs the abstruse and difficult approach to achieve "a sufficient degree of exactness in its sentiments, precepts, or reasonings". Hume famously uses the analogy of the artist and the anatomist. The artist can present a picture to the world that is far more interesting (to most tastes) and far more pleasant than the anatomist can; but the artist can benefit from the anatomist to an immense degree. As he puts it, accuracy is advantageous to beauty and good reasoning to refined sentiments.
(2) Even if nothing more were to be gained from metaphysical discussions than the satisfaction of curiosity, there is nothing wrong with this. Indeed, the life of someone devoted to satisfying his curiosity is, at least generally, inoffensive and pleasant, and even if it did nothing more than this, the person who encouraged it would be a benefactor to mankind. Of course, it does, in fact, do more than this, as per (1).
(3) The most serious objection to metaphysics is that it involves a lot of obscurity, cavil, arguing about words, talking about things we can't know anything about, etc. Hume does admit that this is true of a lot of metaphysical discussion. But he very cleverly turns the objection around by noting that a lot of the cavilling is due to vanity and superstition calling up an intellectual fog. But, Hume notes that this is not a reason for rejecting metaphysics: on the contrary, it's a reason for doing it:
But is this a sufficient reason, why philosophers should desist from such researches, and leave superstition still in possession of her retreat? Is it not proper to draw an opposite conclusion, and perceive the necessity of carrying the war into the most secret recesses of the enemy? In vain do we hope, that men, from frequent disappointment, will at last abandon such airy sciences, and discover the proper province of human reason. For, besides, that many persons find too sensible an interest in perpetually recalling such topics; besides this, I say, the motive of blind despair can never reasonably have place in the sciences; since, however unsuccessful former attempts may have proved, there is still room to hope, that the industry, good fortune, or improved sagacity of succeeding generations may reach discoveries unknown to former ages. Each adventurous genius will still leap at the arduous prize, and find himself stimulated, rather than discouraged, by the failures of his predecessors; while he hopes that the glory of achieving so hard an adventure is reserved for him alone. The only method of freeing learning, at once, from these abstruse questions, is to enquire seriously into the nature of human understanding, and show, from an exact analysis of its powers and capacity, that it is by no means fitted for such remote and abstruse subjects. We must submit to this fatigue in order to live at ease ever after: and must cultivate true metaphysics with some care, in order to destroy the false and adulterate. Indolence, which, to some persons, affords a safeguard against this deceitful philosophy, is, with others, overbalanced by curiosity; and despair, which, at some moments, prevails, may give place afterwards to sanguine hopes and expectations. Accurate and just reasoning is the only catholic remedy, fitted for all persons and all dispositions; and is alone able to subvert that abstruse philosophy and metaphysical jargon, which being mixed up with popular superstition, renders it in a manner impenetrable to careless reasoners, and gives it the air of science and wisdom.
It has been said by others, and Hume argues for it very well, I think, even if he does so cautiously, that you can only get rid of bad metaphysics with good metaphysics.
(4) In addition to this negative advantage of eliminating bad metaphysics, abstruse reasoning can have positive advantages. Even if our metaphysics could do no more than give a 'mental geography', the understanding this would bring would be worthwhile on its own.
The fact that metaphysics is abstract, and abstruse, and difficult, and about obscure and abstruse topics, is a serious disadvantage, one to be surmounted as much as possible. But it's not a knock-down argument against metaphysics like people have often thought. In fact, quite the reverse: metaphysics is still justified even when we give the argument the benefit of the doubt.