Guess I will have to be a bit more serious about my explanation. Seeing as it's now under scrutinization.
Immersion
A tonberry playable race would be a serious detriment to the current and proposed future state of immersion into the game. Tonberry's have always been a minimally verbal race if not completely non verbal in some games. They have almost always been an antagonist to all living things as well. Seeing as they simply hunt people down and kill them.
13-2 allows you to capture monsters and convert them to your side but they are converted there is no inherent urge in the tonberry's created nature to want to assist people. Tonberry's main action and goal seems to be to stab things and watch them die while moving incredibly slow. Thusly in FFXIV the tonberry would require a transformation into a human like creature, because they are removing non-humanoid features from the races. It would also require some lore to turn it INTO this humanoid creature... but not only that it would also by virtue require this method to give them enough time to build a city and society with basic principles and development of battle arts and magic either through cultural diffusion from the primary 5 races or by self discovery. After that, it would require the tonberry(humanoid) race to then develop and foster relations with said primary 5 races as to allow them to freely live breathe and have equal rights in these races cities, while not negating the fact that the 5 races are also highly bigoted and racist against anything besides cat girls (who do in their defense have high verbal skills as well as mostly humanoid forms) As seen in the case of the beastmen tribes, Moogle's Sylphs Amal Ixal Cobal all are fairly verbal but lack distinctly humanoid features to the taste of Eorzean's. So much so that they aren't even considered to be worthy of burial or mercy.
Thusly the implementation of a tonberry race would be highly difficult and they would be an undercast and mistreated segment of the Eorzean population that would be trash to them.
Thusly, if tonberry's suddenly popped up it would annoy everyone due to the fact it would break highly from the common characteristics this game has been appreciated for.
That is why, I said they would do well as a summon, or a lesser beastman tribe (like the sylphs or moogles who aren't hated but still treated with odd regard.) I hope you enjoy my elaboration on my statement since you seemed to be requesting clarification.