Looking at people's accounts of D&D games, I think DMs should not assume players have what I like to call "item luck". Item luck means that all items you get will fit in with the adventure or campaign you are playing. For example, just because you are mostly fighting monsters with weaknesses against fire, don't expect to always get Fire Swords and the like. This applies to both DMs and PCs. A PC should not be expecting item luck, and if he does, the DM should not comply with his expectations.
However, the DM should grant item luck INDIRECTLY. It's clever, cunning, and perfectly reasonable for a DM to give the PCs an extremely rare item that they can sell to buy the item that is strong against the monsters they are currently fighting and/or give the PCs an item that will be useful later on in the campaign or even adventure. Now, at this point, most players will be feeling indignant that if their DM reads this, he/she will not give them item luck anymore, but think about it. In real life, if you were having a headache, say, you really can't expect aspirins to drop randomly out of the sky, right?(if you do, SEE A DOCTOR!)
I once read about a DM who fits this example perfectly. He gave one of his players a Sunblade(I think it was a Sunblade, but anyway it was a fire-based sword). After finding that the Sunblade did not work very well against the monsters they were currently fighting, the player almost sold it. But just as she was going to, bam! The DM took them to a location where the Sunblade was EXTREMELY effective.
So the next time you don't get item luck, don't blame it on your DM(or me). Remember that he is just trying to make the game more realistic and may even be actually giving you item luck, just indirectly. In detective movies, the smallest clue may be significant or may come in handy. The same applies here. See you next time! :D
Note:I was inspired by Dungeonsmaster.com to make regular updates, so I'll try to update this regularly. I can't take all the credit!