Whilest the processor of this type may be more expensive than the cheapest no ROM 80C31 chip with small external EPROM/FLASH

I doubt that, you can get a 6 clocker with 32kb flash for $2.13. OK, just that, no fancy stuff but comparing a chip with ISP etc to a 31/32 with external ROM would not be apples and apples.

http://www.semiconductors.com/pip/P89C58X2.html

Erik