They are, however, not necessary to progress the plot.
I have to agree, Minsc is awesome, but you can progress the plot without him (unlike say a Kreia in KOTOR2 or pretty much any vampire in Bloodlines (Jack, the Prince, Nines)).
Yeah, Imoen is killable. And revivable afterwards too. And if I recall correct the reason why the PC couldn't is because (s)he was a Bhaal-spawn. Imoen is too. For me that is more of an immersionbreaker than (not) being able to kill a NPC.
Although I have to admit I was kind of dissapointed in IWD1 when I used Finger of Death on a kid, and got a X is killed message, and she was still alive...