If you have an interest in computer history, how did we get to where we are now. if you examine a lot of assembly today, much of the time most people are looking at compiler generated code, not hand written assembly. The 6502 is a prime example if a processor to learn. For one thing there are many roms from the classic standup arcade games like Asteroids. You can learn the instruction set by writing a disassembler. Writing a disassembler for a variable length instruction set is a educational programming challenge in and of itself, well worth the effort, or at least like a maze solving program, worth at least getting to the point where you figure out what you have to do to solve it. Because these were hand coded assembler, they didnt have to, and didnt follow programming conventions like a compiler would. Some of these roms either intentionally or on purpose have some traps in them that trip up a disassembler. Back to the disassembler learning experience. Then you can examine the programming style, how it might be similar to general programming in any language and the kinds of tricks and habits used by the programmer for that instruction set. The 6502 holds an important position in computing history, despite the sales pitch leading many to think that Apple had anything to do with anything (they were an also ran behind commodore and tandy/radio shack in the personal computer movement) the 6502 was the common factor in the move from time share computers to ones you could afford to own. As well as the video game world both atari vcs (a.k.a atari 2600) and stand up arcades. Later the z80, and a number of others took over and carried us to where we are now. the 6502 in particular has an addressing mode, which the msp430 can also take advantage of in its own way. YOu may think of the 6502 being starved for registers, but consider the actual registers just temporary registers and the page zero memory being 256 registers.
I also highly recommend going backwards to the 8088/86 to get the history of where the current x86 family was born, compare it to the z80, 8080, and others that sprang from a common source/concept and evolved on their own. x86, even the modern one can be considered a dead instruction set it is so painful and archaic, it just wont die. Why bother translating to some other instruction set inside, why not do what DEC did with the Alpha running windows and dynamically recompile converting the program to another instruction set, allowing legacy programs to run, the reason why microsoft and intel are still in business, but moving away from a poor intermediate interface between the source code and the execution unit(s).
Perhaps the most important lesson is that processors are as similar as they are different, load, store, the basic set of alu operations, xor, add, etc. At the same time even with C, the programming languages never really clinched the features of the hardware, you still have to dive down into assembly to hand tune for performance. core language libraries like memcpy for C are hand tuned asm per platform because the language just doesnt cut it.
You will also find out that many of the processors you claim to be obsolete are still available for sale, sure a small vendor or two with a small market share, but some markets or companies rely on the 8051 for example (have you used a network card lately, sent any TCP/IP packets to anyone? how did you get this web page, there were a number of 8051 driven macs and phys that your packet flowed through, even reading the datasheets and programming those parts you dont realize they are 8051 based unless you have had the history or asked the right question to the vendor when programming those parts). zilog still lives (z80) (and rabbit semiconductor is a modified z80), forms of the the 6502 are still for sale, the hc11, etc, etc.
