It is beyond obvious what each job's role is. What a player can do with them is irrelevant.

Why you would even put so much effort into tanking on a WHM is beyond me when WAR can do it so much better (PLD is meh atm and this patch is supposed to fix it so we'll see) but to each his own. Doesn't change the point that WHM is a healer by definition. It has been since the beginning of Final Fantasy.

So it's not SE telling you to think inside some little box. You can do what you want. SE is saying WHM is a healer by design and thus it is assigned an H. It's more that your putting so much meaning/value on these letters.

I don't have even the slightest sliver of expectation for them allowing us to chose any of the letters for jobs. Maybe in a limited way for some jobs, or maybe full choice for classes. Honestly I see a greater chance they just scrap the letters and simply have 1-8. I'd laugh at any WHM who assigned a T for their letter in a party I'm in though, given we can chose. I wouldn't even be able to take it seriously. It's completely against the definition.

In conclusion, WHM is a healer. It is designed as a healer. It excels at healing. Tanking is not it's strong suit. There are jobs that are suited to tanking and such efforts pay off better on them. That being said, if you have no issue going against this obvious design and building your WHM to tank, then there's no difference between being a guy who tanks on WHM and being a guy who tanks despite being assigned an H.