Glenna Rose wrote:
...........I have started a tomato branch,
rooted in water and planted it, with good luck.
.......................
It seems the suggestion for doing this is to extend the season for
harvesting tomatoes..........................
Is it the varieties I plant (mostly heirlooms) that would make starts
impractical and unnecessary because they bear as long as weather allows,
or am I missing something?.................................

It is your climate.
I would have no reason to do that either, because, here, the season
is so short that some years, my best plants barely produce ripe
tomatoes before the end of the summer. (this may be one of those years)
Your growing season is much longer, but not hot enough to make the
plants stop bearing.
There also may be the matter of determinate vs indeterminate plants.
I guess a determinate plant can pretty much finish growing after is
produces all its fruit. Again, I wouldn't know since my season is short.

Steve (in the Adirondacks)