Let me preface this by stating that it's (mostly) just my opinion.
Anyone who mentions PUP can pretty much instantly be identified as an XI veteran (fanboy / fangirl), since that class exists only in XI. Put plainly, it doesn't have the history a lot of other classes (Red Mage, Geomancer, Blue Mage, Samurai) do, nor is it anywhere near as iconic. Given I got DRK I don't particularly care one way or another, but I'd much rather see legacy classes or new classes than one that existed only in XI before that (especially given PUP was basically just a replacement for Necromancer when they decided not to implement that class). It's a matter of the class having a history or being unique, compared to something like PUP whose only argument for inclusion I can think of is "I liked it in XI."
Why is it I'd rather have legacy classes or new classes? Well, it's important to remember the past, which is why we need legacy (iconic) classes. Some things need to remain constant. At the same time, we need to forge ahead and not be afraid to create something new - creating new classes does exactly that. Leaving rare/unique classes in their own game (such as XI's PUP) lets both it and XIV have their own identity by having something the other doesn't. (MCH, for example, is unique to XIV, so it's "an XIV class," not "a Final Fantasy class.")
Beastmaster falls under a similar problem to me. It's from V, and aside from that game only appears in X-2 and XI, though elements of it do exist in other games (such as VII's Manipulate materia).
Blue Mage, on the other hand, has been around since V in some form or another. Though not the oldest, it definitely has a history and is one of the few classes Final Fantasy can truly call its own (a lot of the classes / jobs in Final Fantasy are almost omnipresent in RPGs). It's iconic to Final Fantasy fans in general, not just XI veterans, so by that token I consider it more worthy of inclusion than something like Puppetmaster.
... I admit I'm biased, and someone mentioning stuff from XI leads me to jump to conclusions. In my defense, from what I've experienced XI veterans are notorious for wanting XIV to be XI-2 in all but name, so it's not entirely unfounded, and mentioning BLU, BST, and PUP (the last of which is exclusive to XI) sets off certain flags. I do try and keep an impartial stance, but I'm only human.
Again, just my two cents.
*readies NulBlaze*