Non-user-serviceability buys smaller and lighter form factors. I would expect the story to be similar for hardware with similar size, weight, and battery life characteristics.
EDIT: and most importantly, more battery chemistry per weight. Safety requires that serviceable batteries are enclosed in rather substantial cases whose internals are non-user-serviceable. Apple "cheats" by making this case the entire laptop, rather than a specific battery module.
Anyone who wouldn't care about having the battery glued to the chassis also would not care about the 1mm difference between making it removable or not.
I also think on at least the Android front Samsung has consistently demonstrated the ability to make phones with both SD cards and removable batteries for years without compromising form factor. The S3 / S$ / S5 and Notes 2 - 4 were all extremely thin profile despite supporting removable batteries.
This is just misdirection to try to persuade people its a good thing to remove choice. It is not, it costs basically nothing in manufacturing or size to make the battery / ram / hard drive removable, and the only reason Apple / Dell / Samsung (now) / every other Android phone manufacturer does it is either to rip you off on overpriced battery replacement or drive planned obsolescence to make you buy more shit you really wouldn't need if you could just replace your damn battery two years later.
So, it's probably wise for Apple or any other company to keep a model or two that are modular to satisfy the geekier crowd. It would be expensive, because with larger-numbers, the razor thin laptops will be cheaper due to scale.
A year ago, I wasn't sure how much longer my ancient (but still functioning!) 2008 MBP would be going on. So I went shopping. Found the same tradeoffs Marco mentions here: I didn't like the sealed nature of the newer offerings (and also, AFAICT no screen density can make up for the inexplicably missing matte displays).
I ended up buying a used 2.6Ghz i7 8GB 15" matte MBP instead of anything new from Apple, though.
About the only complaint I have is that Mavericks seems neither as stable or as well-performing as Snow Leopard, which seems to be the last time Apple released an OS that was a strict improvement over previous releases. Too bad it's no longer safe to run given the state of updates.
I still have my late 2008 MacBook Pro--the first 15" unibody design with a Core 2 Duo. I just spent about $250 for 8 GB RAM upgrade and a 500 GB SSD and did a clean install of El Cap.
It actually runs faster now than it did with the stock HD and Mountain Lion! Granted, I'm not doing a lot of heavy computing with it. For web surfing and the basics it still works great.
EDIT: and most importantly, more battery chemistry per weight. Safety requires that serviceable batteries are enclosed in rather substantial cases whose internals are non-user-serviceable. Apple "cheats" by making this case the entire laptop, rather than a specific battery module.