I dunno about that one. I mean, it makes very good sense, I'm not disagreeing with that. I think it makes sense to have a pet class feed into the pet jobs, but I also think that the pet classes are so different that needing to use one in order to unlock another is silly. Like OP said, I lovelovelove Puppetmaster, and the only thing that would get me to play Beastmaster is if I had to level BST to unlock PUP.
And I think that having a generic pet class that feeds onto all 3 jobs might be a bit boring. I mean, the only kind of pet that would sensibly fit into all 3 classes would be a summoned-mechanical-monster. And to top it off, you would only (as of now) get 5 abilities that are distinct to your particular kind of pet as a job. AND you would only tend to use the pet you really like in party situations as the jobs are designed for party play.
I think a way to introduce pet classes and jobs would be to add a class and job for each.
-Summoner could be based on like an Elementalist class that summons those little elemental guys. And then the summoner can summon more powerful creatures, at the cost of available cross-class abilities.
-Beastmaster could be based on a Tamer class that essentially does the same thing as BST, but BST is designed more for party play.
-And Puppetmaster could come from Puppeteer or Gadgeteer or something that has a Puppet but it is more generic like the Harlequin, and Puppetmaster unlocks the ability to change the frame and head for a more role-specific puppet.
Another way (and a way that doesn't add 6 new classes/jobs), would be to have each pet type be its own class. And then the same pet-enhancing job is availiable for all 3. And then the pet you use is determined by your weapon.
-For Example: I have a lvl 40 Puppetmaster and a lvl 35 Summoner, and since I don't like BST, it is 0. I can do the job quest for a Mentor job or Familiarist job. So I unlock Mentor as puppetmaster and summoner, but not BST because I lack the prerequisite levels.
-As mentor, I can equip H2H weapons and whatever summoner uses (bows or wands or something). That weapon determines what pets I can use as well as which base class abilities I have access to (all of puppetmasters abilities if using H2H, etc). Then the mentor class abilities tend to enhance pet abilities, regardless of pet type.
This way you can have the pet type you want from level 1, rather than unlocking that pet type at level 30 and only being able to use that pet type if you are in job mode.
Sorry for the long post, I just think that Pet jobs are going to be tough to implement well in the current class/job structure.