Why is The Monkey`s Paw not a desirable gift?
First off, I wouldn't call it a gift. The paw is passed from person to person. In that way, it comes into one's possesion; it is not given as a gift. The sergeant-major got it from a man who wished for death. So it was from that man that Morris (the sergeant-major) came to own the paw.
Now it is the central idea of the story that there is really no way of knowing for sure that the paw has any super-natural powers of wish granting whatsoever:
"Morris said the things happened so naturally," said his father, "that you might if you so wished attribute it to coincidence."
Certainly, after one makes a wish with the paw in one's hand, something happens after the wish. But there is no way to know if the paw granted the wish or if what happened after the wish was made would have happened anyway. Things always happen after other things; that's the nature of time. The last wish made, before Morris got it, was a wish for death. But we really don't know if that man, who asked for death, even died, although that's what's implied. But so what? People die. Then Morris got the paw, and we don't know what his three wishes were at all. At least he's still alive and seemingly OK. Because of Morris's attitude about the paw, however, we assume he thinks it does bad things. Indeed, he throws it into the fire.
The new owner, Mr. White, makes three wishes with the paw in his hand. The first wish is for money. Then the family gets some money. Is it because of the wish? Who knows? And that's the point of the story.
So, now to answer your question: is the paw something desirable to own? Sure, why not? Should you use it to make wishes? What do you think? And that's the point of the story.