It is very interesting to follow the codices as they evolve. Minute changes are made to existing units, while new units are added. Very rarely is a unit removed from the codex.
Following my last post on the codex timeline, I've been thinking about this codex evolution, especially concerning new models. Very often, at least the last few years, something that was crap gets a litte boost, and something that was a no-brainer gets nerfed. It would seem that units that get new models with the codex (Lootas, Battlewagons, TH+SS Terminators, Leman Russes, Carnifexes, Trygons, etc) gets a little edge over other units. (This is clearly not always the case , but I think my point still stands.)
A great marketing ploy, no doubt, but also one that makes it fruitful to try new things. It would be pretty boring if Assault Marines had been a no-brainer choice for the last 15 years. Making new and powerful units is really keeping the game fresh for you veterans out there!