The rationale is that within autism there are many different presentations all meeting autism criteria (something that PDD-NOS by definition does not do), with a mix of aetiologies as well. AS research carries on we can gradually start to pull apart the causes and types and better understand those individual disorders, 'autism's will just be a header for those different disorders.
So using my kids as an example- ds1 and ds3 both have spectrum disorder severe enough to not be in MS education: ds1 has AS, ds3 HFA. however they are really dissimilar in how that presents and their needs- ds1 is quite classic AS and has aggression issues as well. DS3 is not typical HFA becuase despite a 'HF@ IQ rating his functional level is lower- he has a lot of obsessions, big issues with absences and is unlikely to be independent. but he is also social- he just ticks the inappropriate box by being TOO in your face, indiscriminately huggy etc. According to DSM criteria they both meet the ASD definition and that's right and proper given their shared genetic history (something that is a few generations in our family in terms of ASD) but their needs are not the same and an umbrella term covers that far better than labels that don't quite fit.
We also talk increasingly of a quad of impairments now LOL as well, the fourth sector being sensory. All in it's infancy (uni course is MA in Autism) but i think the quad aspect is especially positive.
The asd 123 thing might be a useful tool for planning services and allocating funding but there needs to be something that operates at an understanding level within the wider populace. 'Autisms' does not require a specific diagnostic knowledge. It's also pertinent that functionality within ASD is not always impaired by the diagnostics alone, eg ds3's absences (not epileptic)- so a descriptive of the type you emntion still would not cever his needs in any real way.
PDD-NOS is not the same as PDDs. The NNOS is a specific term stating that the PDD do not fully fulfil the criteria set for other specific diagnoses. ALL children on the spectrum and with a range of other disorders would fit PDDs as they do inded have a pervasive developmental disorder; it is the NOS bit that matters as a label.