They aren't as easily serviceable as older Thinkpads. For instance,
replacing the battery now requires you to open up the back cover and
unplug a fragile ribbon cable, while with older Thinkpads (like the
X200), you can simply slide a new one in. Pretty useful when you want to
swap in battery on the go and you have a spare in your bag.
In some cases you can even replace the CPU and RAM (like on the T440P
which I currently use). If I wanted to, I could upgrade to a quad core
to get additional performance in many workstation tasks without getting
a new laptop entirely.