You know how Ragnaros was a complex fight made of many parts and it was difficult when it was new right?
Imagine that something similar may happen if 3 or 4 "easy" bosses are put into a "chain kills" mode and by killing one you're automatically to the next and if you fail on any of them you fail in all.
After all, if you only started with the last phase of Ragnaros, it would be relatively easy, if you had all the time in the world for Algalon it would be much easier, even if you started on the 3rd phase of Ragnaros it may have been a much easier endeavor.
That makes me think Yogg'Saron was one of the most amazing encounters ever created. This is because while the first phase was a similar "grind", the 3rd phase was not as important as the 2nd. If one only had the 2nd they might still fail. Of course it wasn't the same to have just that but the difference would not be as huge, as compared to say, getting only the 4th of Ragnaros or even starting from the 3rd.
But anyway, on the chain-kills of any raid it makes sense. 2 or 3 "easy" fights can become insanely hard if one must do them without wipes. This might not seem a good thought for Dragon Soul now but imagine that when one is on progress there will be wipes even on the easier bosses. Even if on farm you had one wipe in any of them it might mean a wipe for all. It wouldn't be as easy even on farm.