I am an ex-jetstream QFI and have also flown the seneca during the ATPL thing. The Jetstream is a much more demanding aircraft to fly in terms of basic handling and also systems handling/knowledge and when i was there it was a logical progression from the Tucano for most students. By comparison the Seneca is a 'Ford Escort with wings' but i guess that since most students will be coming from a non-turboprop/jet background then it is a logical and more manageable step which seems to work for the airlines. Although formation and low level flying were included in the course syllabus, these trips were frequently dropped to allow extra procedural training in the hours available.