The simplest reason for not allowing sharing a cable between applications is that applications change (much) faster that cabling system that's inside the walls. The trend is to go for more speed all the time, and that requires more pairs for simultaneous multiple channel data transmission as in 1000BASE-T Gigabit Ethernet. We do have enough customers today who wish they did not insist on splitting those cables because they are practically banned from deploying Gigabit Ethernet or they have to sacrifice phone, which is not always possible. In the nearest future more apps like Gigabit Ethernet may come up, and the need for all four pairs will be even stronger.
There are also quite a few older apps that require three or all four pairs just because the interface requires more signals for communication, such as RS 232.
Dmitri Abaimov, RCDD