The way Specialized has changed the Enduro over the years, they might as well have given it a new name. However, they're smart and realize that customers have built up brand familiarity with the name "Enduro" and rather than scrap that and start over every couple years, they use the same name on the new bike design. Think about the 2000-2001 Enduro, which was replaced by the monocoque 2002-2004 Enduro, which was completely redesigned for 2005 with the split seat tube design, then was scrapped again in 2007 for the Enduro SL design with the vertical shock and rocker link. That's a huge amount of change every 2-3 years, but the name remains a common thread.
Ultimately, the difference is not in what the companies are doing with the actual bike designs, the differences are in the ways the companies understand brand loyalty and the marketing value of continuing a specific brand lineage. I think you're right about Cannondale confusing customers by introducing new brand names all the time. While on one hand it does inject a new energy into their product line, they're ignoring the valuable brand equity they could build up in their product names.
However, the move with the new Scalpel is a positive one in that regard. Despite the radical redesign, they smartly kept the same name. Now if only they'd called the Rize the Jekyl...