I'll give you the stat buffs being useful if there are 8 encounters within the space of 10 minutes on a regular basis. (And party members don't have stat increasing items). However, that's not been true of any campaign I've ever played in--even the Return to the Temple of Elemental Evil rarely featured more than three encounters in that space of time--and that was if we were hurrying.
You and I clearly have different ideas about and experiences of the "generic dungeon crawl." IME, a "go, go, go, we've got to get to the next encounter!" type approach in a dungeon with no traps might get through two encounters in six minutes--three if you're lucky. You also seem to expect that the majority of campaigns are generic dungeon crawls which is not my experience or expectation either. For a city adventure involving some investigation, a wilderness adventure, or any campaign that involves the risk of ambush when travelling, min/level spells, extended or not will usually only last one encounter.
Where did he do that if I may ask?
That's not what this thread is discussing but if you ask me the revised stat buffs were never clearly better than every other 2nd level spell on the list (for clerics, Remove Paralysis, Lesser Restoration, Silence, etc were always competition and for wizards, glitterdust, web, aganzzar's scorcher, see invisibility, invisibility, and rope trick were always competitive as well). The revised stat buffs are clearly competitive only with other loser 2nd level spells like Aid and Blur and good first level spells like Shield of Faith, Protection from Evil, Bless, Shield, etc. The choice isn't tougher now; it's easier. Never prepare a stat buff spell unless you're in a party willing to speed through a dungeon on the and have reason to expect that you'll have lots of encounters within the space of 3-8 minutes and no traps.
Of course, the only time that extended statbuffs are advantageous is when the PCs have the same expectations but expect those encounters within 10-16 minutes instead of 5-8 minutes. If the PCs expect the encounters within 5-8 minutes, then extending the spells is a waste of a slot; if they expect them within 17+ minutes, extending the spells doesn't help.
This is another argument about reduced duration rather than about the usefulness of extend spell in the new edition but I'll bite.
Not necessarily. I could very easily write a game where players have to put lots of prep and thought into every combat or they'll get their hides tanned. I don't think that making monsters so much more deadly that PCs have to put lots of prep and thought into every encounter would make a game more fun though. And, even then, it's not what the change has done. The reduced duration didn't make prep and thought--formerly useless commodities in the 1 min/level duration--suddenly useful. Prep and thought on such short term matters made a huge difference already. The difference between a party going into (IME one) encounter with Bless, Shield of Faith, Shield, Protection From Evil, Protection From Elements, and possibly haste prayer, etc all active and going in with only hour/level buffs up was tremendous. What the reduced durations have done is remove a large category of "prep and thought." Specifically, they have dramatically reduced PCs' ability to be ready for the unexpected. Having buff spells working 24/7 as was possible under 3e was "prep and thought." An 8th level caster spending a 2nd and 3rd level slot to have have Cat's Grace active all day did so because he knew that lots of things happened which he could not anticipate and therefore prepared for unanticipated trouble. In 3.5e so far, such "prep and thought" has been reduced to Endure Elements, Detect Scrying, Mind Blank, Nondetection, and their ilk.