[Drammach]

If comets are so loose why would we need to divert one from a collision course with Earth?

If the comet's big enough, and enters earth's atmosphere at the right angle and speed, it superheats and explodes..
Think Tunguski, Siberia..

Are we talking about this particular comet or all comets ?

Depends on who you're talking to..
Some believe this evidence will hold true for all comets..
Others, that further evidence is needed before we can say this is evidence for the composition of all comets..
Until then, it is only evidence for the composition of thise "one" comet..

One other thing..
We just provided this comet with a solid copper core..
Not a very big one, and it may have vaporized, we aren't sure.. ( but even if it did, that copper is probably still part of the comet now.. )
We will have to see what effect it will have on the comet's composition..

[Junior]

Some believe this evidence will hold true for all comets..

The Leonid meteor swarm is supposed to be the rocky debris left by a comet. One gets the impression "comet" might be a catch-all term for "objects composed mostly or entirely of gaseous material in highly elliptical orbits."