This is a bit condescending, and also a generalisation based on (as far as I can tell) no evidence.
My impression, both from posts on these boards and from the popularity of Paizo's APs, is that the use of published adventures is very popular, and not just among inexperienced GMs.
I have been GMing for nearly 30 years and use published adventures for three main things: (i) maps; (ii) ideas for story elements (eg interesting antagonists); (iii) ideas for engaging situations (eg interesting encounters). As Lanefan has talked about upthread, I do a lot of mixing and matching of this material.
From 1st to 16th level, my 4e game involved a mix of story elements and maps from seven modules: Night's Dark Terror (an old B/X module); Thunderspire Labyrinth (a 4e module); Speaker in Dreams (a 3E module); Heathen (a Dungeon adventure for 4e); Sceptre Tower of Spellguard (a 4e module); The Bloodtower (a mini-module from Open Grave); Wonders out of Time (a d20 module by Eden Odyssey). Many story elements were adapted or combined (eg the principal town in the campaign combines Threashold from NDT, Adakmi from Heathen, and Bridonford from Speaker in Dreams; NPCs are combined, developed, etc). I also draw some maps of my own, and come up with my own story elements and backstory.
I've been running campaigns in this way for around 20 years.
I would not want a new player to start with any kind of published adventure product. I would want them to bring their own creativity to the table, which is infinitely greater than anything you'd get out of any published adventure.
I'll just mention one aspect of the game: maps. Exploration-focused play can benefit from interesting maps. 4e combat encounters benefit from interesting maps. I draw some of my own maps, but there are plenty of people in the world who are better mappers than me, and I'm happy to pay some of them for their labours!
In my experience, creativity is not equally distributed among the playing population either. To point to an example that I don't use myself, many RPGers are probably less creative than Greg Stafford (designer of the Glorantha world for RuneQuest and HeroWars/Quest).
To point to an example that I
have used, the backstory for the TSR adventure OA7 Test of the Samurai is not something that I would have come up with myself: a dragon and animal lord, exiled from the heavens for an indiscretion, finds himself aging due to his lost immortality; he therefore comes up with a scheme to practice a regimen of training that will permit him to become immortal if he breathes only yun ch'i vapours ("vapours of death"), and sets out to replace the world's t'ien ch'i vapours ("vapours of life" - air) with yun ch'i.
The scenario to AO7, as written, is pretty railroady, but that is easily ignored. It has a number of memorable NPCs who were easily adapted and incorporated: Za-Jikku (the fallen animal lord) himself; the Peachling Girl, who leads a rebellion against an oppressive provincial governor; the peasant who found and mastered the Book of Immortality; a snake cult; etc.
There are other creative D&D modules out there too, like OA3 Ochimo, the Spirit Warrior (also has some nice maps), and The Bastion of Broken Souls (another module with interesting backstory and NPCs, coupled with railroady advice to the GM that is easily ignored, and a tedious dungeon crawl in the second half that is also easily ignored).
Drawing on backstory and story elements conceived of by someone else is no obstacle to creativity in play (from both GM and players).
the DMG needs to have a section devoted to adventure design
There is such a section in both the 3E and 4e DMGs. The 4e one is not all that good; I can't remember the 3E one very well, but don't recall it being excellent either.
There is also a brief but quite good discussion in the Moldvay Basic rulebook.