So here we have two people, Glycerine and Teggy, who did the grav gun thing on their first run through Ravenholm. (I'm making an assumption for Glycerine since he said he got the achievement and doesn't replay to get achievements he missed.)
On their first and possibly only run through Ravenholm, both of them didn't once get to blast zombies with a shotgun. It's a really fun game experience, and neither one of them got to do it.
I played through most of the game on the PC years ago, and had plenty of opportunities to blast zombies with the shotgun. This was my first and only run through the on 360.
But besides the fun of the shotgun, there's the developer's intentions for the game. The designers clearly played up the shotgun vs. zombie fun, not only by making the weapon available for the first time in Ravenholm, but by having it's arrival heralded by Father Gregorio's "Let me get you a more suitable weapon." Great gaming moment, and yet both Teggy and Glycerine immediately stuffed the more suitable weapon in their pocket because they were playing by the achievement how-to-play checklist.
And your point is? What makes the shotgun more suitable? I remember playing that level on the PC and exhausting the shotgun ammo way before I was any where near the end. Granted, you do find more, but the GravGun doesn't require shells and thus can be used as many times as necessary to get the job done. Besides, I really got a kick out of slicing zombies in half with the plethora of saw blades just laying around the level. To me, that was much more fun than blasting away with the shotgun, which was pretty much the same as every other game all the way back to the original Doom.
Yes, the designers also made the level a grav gun playground. But that should just be an available option of how to approach the level instead of the standard way to run through. Indeed if you look at Gamefaqs there's a 2005 FAQ on how to play through to the end of the game with just the grav gun, so people have certainly already opted to have fun that way. The difference is that the 2005 players chose to ignore the shotgun moment out of their own volition, not because they had a checklist on how to play each level 'properly' for the most efficient point collection.
Who said it was the standard way to run through it? To be honest, after playing the same level on PC, I didn't think there was any way in hell I could make it all the way to the end with just the GravGun. It sounded like something fun to try, especially since I hardly ever used the damn thing before. Much to my delight, I discovered all kinds of ways to deal death with ordinary objects. In the end, I actually enjoyed the experience much more than running through and blasting everything in site with whatever gun I happened to have ammo for.
And yes, Glycerine and Teggy weren't forced to play the way they did. But they were enticed by the achievements as they are designed to entice, and if someone wants achievements it seems quite natural to want to collect as many as you can on your first run through to collect them more efficiently. Thus, the achievements become the game, or at least the instructions on how to play the game. You lose both the game creator's vision for a scenario like Ravenholm as well as the freedom to find your own path.
If they really were the game than I'd have 1000/1000 score on TMNT, or some other asstastic game with easy achievements. As it stands now, the only ones I have for TMNT are from my 9 year old who absolutely loved the game. Achievements for me are like a history of my gaming accomplishments, I can look back and remember getting my first ever for Dead Rising. Looking through any particular game on my list allows me to fondly reminisce a bit about my play through. They also, at least when they are inventive and well thought out, invite you to try and do some things you might not have thought of. Your assumptions, while intelligent and well communicated, are way off.