Even if I do say so myself, The Post adds at least a little credibility:

Keeping Windows To the Past
Experts Say Modern Doesn't Mean Better
By Kathleen Howley
Special to The Washington Post
Saturday, March 11, 2000; Page G01

...John Leeke, a preservation consultant based in Portland, Maine, said the
worst possible scenario is to replace original wooden windows with plastic.
"If you hire an ordinary remodeler, he will knock out your old windows and
replace them with plastic. It's like the 'tin men' who sold aluminum siding
to everyone in town a generation ago. Even the old-house fanatics fall for
it," said Leeke.
Leeke is a man on a mission. He travels around the country lecturing and
giving workshops on a variety of topics related to the preservation of old
homes. Windows are high on his list.
"I've seen windows that just had to be cleaned being pulled out of houses
and plastic-framed windows put in. It's a significant loss to the homeowner
because it's a loss of value. It doesn't make too much sense to replace a
wooden window worth several hundred dollars with a plastic window that is
worth about $80," he said.
Leeke said he is not opposed to plastic windows in all cases.
"They are not all bad. They just don't preserve the character of old
buildings. And when they break, you have a problem. The only thing you can
do is buy an entire new window because there are no craftsmen who can repair
them," he said.
Leeke said putting double- or triple-insulated panes in old wooden window
frames is not the answer, either. First, it means the mullions likely would
have to be replaced to accommodate the thicker glass.
But, more important, the insulated glass doesn't last as long as single-pane
glass, he said.
"The seal between the panes of glass leaks, moisture gets in and it clouds
up. So you're throwing away all your good glass, knocking it out and
replacing it with double-insulated glass that is going to fail in 10 years,"
he said.
The newest generation of insulated glass is predicted to have a lifetime of
up to 18 years, said Leeke. Still, that doesn't impress him.
"A single pane of glass will last until it gets broken. We work routinely on
windows with glass that is more than 100 years old. We repair the sash and
put the glass back in," he said...