I totally agree with Gilvan Blight.
However, there is one key element that i feel has not been addressed in this discussion. Are we talking D&D 3.5? The reason i ask is this... IMHO the new versions of D&D exist as a means of hack and slash meat grinder type of play. The amount of Role-playing as opposed to Roll-playing that you can do with the current set up is so amazingly limited by the design and mechanics of the game that it\'s hard to see how much \'eb and flow\' of a game you can have without getting down to \'hack and slash\' at some point.
I\'m not saying that imagination can\'t fill in the gaps a little, i\'m simply pointing out that the characters are built in D&D as monster killing machines and little else (possible exception of the rogue and bard). The mechanics of the D&D system drive toward combat, combat, combat. Role-playing type feats are viewed as a waste by most players i\'ve encountered. As further evidence how much of the DMG is devoted to deeper player character development and story telling, as opposed to mathmatical calculations, and monster/player killing mechanics?
If you are playing a system other than D&D (D20) that lends itseld to more of a balance of Role-playing vs. Roll-playing, then there is no need for modules, as the exploration of the characters that have been created is a significant purpose of the game. However, if you are playing D&D 3.5 then why NOT use a module? The point of the game is hack and slash. At least with a module you know it\'s balanced, you can push the monsters as hard as they can go at the party, and the players know they are in for some combat and can plan accordingly. It\'s D&D at it\'s best (sad but true).
I\'ve run for many years (18+), i now exclussivly run modules for D&D because i choose not to reinvent the wheel. I can spend my limited hours drawing up maps, scrutinizing over the appropriatness of the Encounter Level of 2 Ogres vs. a Minotaur for my players current quest. Or i can grab a module and get right down to the business of providing my players with the maximum challenge that the module allows, with the comfort of knowing that the module (story arc) is complete and my players know they are in for a reasonably fair fight.
Bottom line, anymore D&D need modules as much as it need miniatures. The game is masterfully about hack, slash, and treasure grab. Asking it (D&D) to do anything else is unfair to the players and to the system.
D&D D20 Modules... YES!
Other systems... maybe?