If gen 2 gets split in half, we should also have a Gen 4.5 for the early 90's. The 16-bit era began in 1987 and 1988 with the PC Engine and Genesis, so certain systems released from 1991 to 1993 stand out in terms of power and types of technology used, especially CD-ROMs. These include the Neo Geo, 3DO, Jaguar, CD-i, Pippin, and arguably the 32X.
I believe the term "generation" implies the existence of a healthy market. Since the Atari 5200 and Colecovision failed to create that kind of market, they are grouped with generation 2, and are considered a failure to make the true leap. Likewise, the 3DO and Jaguar failed to ever be part of a healthy console market, and are thus not considered part of the 5th gen proper.
The borderline between being a part of a healthy or failing market is a bit hard to make out. Though the Dreamcast sold well, one can argue that its 1998 release and short lifespan make it as much of a 5th generation console as a 6th generation console.
Ultimately though, the entire concept of a console "generation" is more psychological than actually based on hardware. Generations describe what other consoles a system was competing with on the market and in the minds of consumers. The reason the Atari 5200 is considered part of the second generation is that it was never able to find its own spot, not already taken by 70's era machines.







