A curse is not dependant on the one who made it to be still alive - or even to be very powerful. We have both the curses of Isildur and of Mim working after their deaths, and none of them, esspecially the dwarf, can compare with Melkor in might. I believe this has to do with the magic of words, to which Tom alludes when talking to the hobbits. Words have been often use; we see Sauron defeating Finrod in songs of power and making the One Ring, again with the use of words; we also have the words of command of Gandalf, and the songs of the barrow-wights or of Tom - I am sure this art is taken to another level by Melkor, who corrupted all the matter of Arda, getting a grip on all embodied beings. This grip may be more powerful than using curses on particular beings - so it may not worth the time spending doing them. Unless we talk about Hurin
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