It makes no sense either way to have a infinite value here, so +1 for not using it
Ok lets see, if I can explain.
You need to take into account the difference between TCP keep-alive and HTTP keep-alive feature, they are NOT the same.
IS 6.5 did support only HTTP 1.0 when acting as a server (This is when you define a port), and some HTTP 1.1 when it was the client.
IS 7.1 supports HTTP 1.1.
The keep-alive setting is part of HTTP 1.1, so that’s why IS 6.5 did not have this setting.
What does HTTP keep-alive do? Please check http://en.wikipedia.org/wiki/Keepalive
Also notice Apache advice on this setting: http://httpd.apache.org/docs/2.0/en/mod/core.html#keepalive
That’s why my advice was not to set a infinite one.