[monocasa]

They are usable externally. The Realview boards use it to connect a CPU chip with an external northbridge, but it's really rare. Even in that case, it's just because the Realview board is meant to prototype SoCs with the help of an FPGA.

[duskwuff]

Usable, but only barely. As AXI / APB3 are intended for on-die communications, they lack the line coding and error correction used by other busses like PCIe or SATA. On the upside, though, they take a lot less area to implement.

[monocasa]

That's more a distinction between serial and parallel buses than on-chip/off chip. If you're not pumping a single signal to gbps speeds, you don't need the clock recovery and ECC that serdes style connections imply.

It all depends on your use case.