The argument is theologically moot.
Since Jesus has supposedly taught on the model prayer: "Let your name be sanctified", let us remind what is implied by the term 'sanctified'.
The Greek word is Hagiasthētō (Strong's 37). From Hagios: means "to make holy', 'purify', 'consecrate' (Strong) 'to render inviolable', 'to purify', 'to cleanse externally' (Thayer)
Since we can agree on the following:
a) No one knows what is the original and correct pronunciation of the tetragrammon; and
b) It is well known that the name "Jehovah" is an interpolation of the letters of the tatragrammon with the vowels of 'adonai', concocted by a catholic monk in the 13th century, and, therefore, it is impossible that represents the correct hebrew pronunciation;
It stands to reason that, as per our current knowledge, it is impossible that "Jehovah" is the original, pure, hebrew pronunciation of God's name - the name that the hebrew God has chosen for himself. Remember that no man can question God's choices or attempt to change them. To Isaiah he said: "Yes, from ancient days I am he (...) I act, and who can reverse it?" (Isaiah 43:13)
As per God's own statement, no man can reverse or pervert God's choices, including the choice of name and way of pronouncing it - in hebrew - that he picked for himself back in the days of the patriarchs.
Therefore, calling God by the name of "Jehovah", even if well-meaning, it has the effect of polluting, corrupting and bastardizing God's name. Isn't it the WT that insists at pointing to Galatians 5:7: "A little leaven leavens the whole lump of dough" to say that even small amounts of corruption are enough to make an entire doctrine morally objectable to the Witnesses? The poor Uzzah thought he was doing something good by taking hold of the covenant ark when it was in peril of falling from a cart. In his eyes, it was a good deed. But God struck him, because he committed an intolerable act of impurity, since he wasn't a Levite of the Kohatite branch. (2 Samuel 6:1-7) That's how stern God is with purity, according to the Scriptures. Don't angels sing "Holy, holy, holy are you?" (Revelation 4:8) Well, the WT should heed to their own advice on this key doctrine.
Even if the WT could be on the right side of the theological wars by noticing the need to rescue God's name from obscurity, doing so without being absolutely certain that they were using the original and purest form of the name, would risk them being the object of God's ire.
To insist on using "Jehovah" is actually a disservice to the God the WT purports to worship, and, if I were the God that the Scriptures portray, I'd be very unhappy and offended with them. It's a bit like if my name was JONATHAN and a bunch of people at work would call me JANITOR because it was a "close enough" pronunciation. I wouldn't be amused. I would rather be addressed as "Mister" or "Sir", than be called some corrupted form of my name that, after a while, I would find an intolerable mockery.
Besides, I don't see any advantage in God allowing the true pronunciation of his name to be concealed for millennia. What good would that produce, except to expose God's inability to preserve his own assets?