The Monk is designed to be a DPS with off-tank abilities. I'd expect the Monk to be able to tank for a group in a pinch, but to far less effect than a real tank. Even with the best gear they can find for "tanking".
At no point should a Monk be able to tank as well as a true tank class. This same should be true for all classes.
Like many other subjects, there is no black & white answer for this. It's all a matter of degree.
VR has been pretty consistent in the position that no Class can be generally competitive for a spot in groups while playing a role other than their primary one, regardless of gearing. Yet most classes have some abilities that fall into a different role than their class's main role, and can do a bit of short-term 'filling in' should the main person in that role be incapacitated.
Early levels are also not expected to require 6-person groups with all 4 roles represented, for every situation. Some non-hardcore players may choose to play this way thru as many levels as they can. I'm sure some will try. How far up the levels such an approach can get you remains to be seen. I doubt those players will be in very many raids, unless enough of them form their own Guild.
I don't believe VR is - or should be - putting much effort into creating situations to do this. They are aiming for strong Class & Role identity as a major attraction of the Game. If you can find a way to expand your character beyond that, good on you. VR calls it 'emergent gameplay'.
Another consideration is that serious effort to play your char in an 'alternative' role would include putting points into different stats than you would for your primary role, and putting Mastery Points into abilities that aren't the main ones for your primary role. Thus you would have a char that couldn't just switch his gear in order to return to full functionality in his primary role. That would be a lot of time and effort invested in a char who only was played "in a niche situation".