To a large majority of JWs it wouldn't matter. Most Witnesses don't understand JW chronology. They just roll with it.
The JWs don't need 607 to support their belief that the "last days" began in 1914. They just point to WWI starting in 1914 and then make some vague reference to "worsening world conditions" to prop up their end-times chronology.
But to me, even more damning than the 586/7 date for Jerusalem's destruction is the lack of any scriptural support for a second fulfillment of the prophecy. Where in Daniel is there any mention of a second fulfillment? I don't see it. Where is it in the NT? I don't see it there either. (I know that JWs point to Jesus' statement in Luke about Jerusalem being trampled by the nations until the appointed times of the nations are fulfilled...please! That single statement is supposed to tie an already-fulfilled prophecy in Daniel to something in 1914...really? You can't be serious.)
If you want to debate 1914 with a knowledgeable JW (which is often--but not always--an oxymoron), I recommend not focusing on the 607/587 issue. You can get bogged down in the details. Rather, I would have the JW explain (1) where the Bible says there is a 2nd fulfillment of the prophecy; (2) where it says the 2nd fulfillment should be calculated using a day for a year; (3) where the Bible says to use 360 day years to calculate the number of years but to use 365/6 day years to calculate the end of the period.
The way I see Daniel's prophecy is this. Old Testament God is a bit full of himself, and he wanted to teach Nebuchadnezzar a lesson in humility. So, God predicted that Neb would go crazy for 7 years and then be restored. According to Daniel, this happened. Thus, the prophecy was fulfilled and God was shown to be the big shot that he is. End of story.
Of course, that's not enough for some people. Over 2000 years later, the Adventists and Watchtowerites conjured up this idea that there must be a second fulfillment that would take place in their time. Thus, they shoved the round "day for a year" peg into the square hole and came up with 1914. It's laughably absurd when you take a step back and look at it.