For external XRAM, it isn't enough to have the interface. The important thing is to have the interface and still enough pins for any other task needed. A 40 or 44-pin chip may become a "low-pin-count" chip for the user after the addition of external XRAM. This problem isn't unique to the 8051 line but affects any processor. Many other architectures don't have support for multiplexed memory until they add support for DRAM, so some ARM may loose very many signals for accessing external memory. An NXP LPC2388 for example would consume 28 pins for 64kB external SRAM - and still only get an 8-bit memory interface for a 32-bit processor. Built-in really has a lot of advantages, but luckily there are 8051 chips with more than 40/44 pins.

Good to know about the gotchas with some of the chips claiming 64k+ of XDATA in Keils database. I have always selected other processor families when in need of more RAM, and most 8051 work is to keep alive older designs.