While you can't say Kaddish without 10 men present, it's possible that not all of the other men will make the appropriate responses (whether oblivious, ignorant or apathetic). Another case would be where two or more people are saying Kaddish at the same time and part of the audience is responding to one person and part to another.
What does it mean for the effectiveness/validity of the Kaddish if a minyan doesn't answer? If you are certain it will happen should you rather not say it?
Michael Sandler, you have 2 questions:
(OK, maybe that progression should have been the other way around.)
A "davar shebekedusha" is a matter that requires a special presence of G-d (from the pasuk of Hashem nitzav ba'adas e-l we learn that this is 10 adult men). Once Hashem's special presence is assumed, the required kaddish may be said regardless of whether people are listening. The Aruch haShulchan (OC 55:12) uses the above idea to justify why one who can speak but not hear may be counted as one of the 10.
In contrast, the Shulchan Aruch (OC 124:4) states that by the leader's repetition of tefila, if 9 are not paying attention, the brachos are almost as if in vain. I would guess this is because kaddish is a requirement even for 1 person, but there is a technicality that it can only be said among 10 (see end of Aruch haShulchan 55:9). The repetition of tefila is created only through having 10 people, so ten congregants have an equal obligation to say the repetition, a requirement fulfilled by the appointed leader- the shatz.
Note that the Shuchan Aruch's requirement by the repetition (124:4) just says they have to pay attention. Saying amen in not a requirement of the congregation, but a personal requirement that when we hear a bracha we should say amen (see Aruch haShulchan 124:11). Even if one is fulfilling his obligation through the bracha and the amen becomes more pertinent, we say shomea k'oneh (listening is like answering).
