Commit 516ffb6b says:

Stephen Hemminger [Thu, 22 May 2008 20:41:40 +0000 (13:41 -0700)]
> Use the standard POSIX inet_pton to convert from string to IPV4
> address. This avoids problems where ip parses "127.2" wrong.

Apparently inet_pton doesn't support abbreviated/shortened/classful
ipv4 addresses at all, but inet_aton does.
Since the function only deals with AF_INET anyway maybe using
inet_aton "to increse backwards compatability" (please those
who still want to use the format) could be considered?
(This will still not restore the 10/8 format which apparently used
to work in iproute, so people would have to settle for 10.0/8)

(See http://bugs.debian.org/497011)