Of course he [Luffy] should train to fold it into his repertoire as cleanly and effortlessly as possible. Just like Sanji should do with Diable Jambe and Zoro with swordsmanship. I doubt there will be anyone on the crew at this point who fights with their basic physical strength.
Considering that they, presumably, don't have any physical enhancing abilities, how do they not use their basic physical ability when they fight?
Luffy's Gear Second are fundamentally different from the Zoro's swords and Sanji's Diable Jambe though. It's nature is a plain boost to his physical capacity, and thus ought to propel him above his peers in that aspect, rather than simply compensate for Zoro's swords or Sanji's flame kicks: the augmented attack power Luffy receives from his stretching momentum does that already.
I would have been happy if Luffy had taken a page from Zoro and Sanji, and developed combat techniques (e.g. Rokushiki (not necessarily them specifically, but similarly unique techniques)), rather than a plain physical enhancing "mode".
The above page and a few others throughout the chapter seem to imply that all Gear 2nd does is advance Luffy's body to the point where he can use Soru. […] Does Luffy really even need to use Gear 2nd to use these abilities anymore?
Gear Second increases Luffy's physical capacity: he becomes stronger and significantly faster and thus he was able to overcome his lack of skill and use Soru before the time skip.
Though we have yet to see it, I'm certain Luffy can use Soru without physical boosts after the time skip (would be disappointing if he could not).
so do u think Sanji and Zori are haki users now,or not?
I certainly think so. If they can't utilize Haki, but still remain comparable to Luffy, it would suggest that they has a significant physical edge over Luffy (including Gears), or, alternatively, was outstripped by Luffy during the time-skip. Personally, I can't see Oda going either route, especially as there is no reason why they can't have learned it during the time skip.
Besides, I can't really see Luffy spending the next 2-4 years teaching them the trick (it took Luffy, an alleged Haki prodigy, one and a half year of personal tutelage from the Dark King himself to get the forms down).
Well, Kizaru and Whitebeard both gain obvious physical boosts through their DFs, but that doesn't seem to bother anyone.
^Probably because neither Edward nor Borsalino gained any "obvious" physical boosts through their Devil Fruits. Edward had gargantuan attack power through his quake ability, but it didn't change his physical ability. And while Borsalino can supplement his speed with his fruit (e.g. his kicking technique), it doesn't change his physical speed.
But at least not nearly all of the main characters are suddenly able to do it. […] Because it, again, means that only Luffy can pull this sort of power-ups.
Yah. Only Luffy are able to use Gear Second, and only he (apparently) needs the physical boost it provides to compete. How exactly does that make it more appealing?