They are needed. Game is to easy without them.
I havent played the game long enough to finish. But I have restarted it a couple of time. And I must say I know EXACTLY how to kill with ease.
The only zombies that are an issue for me is Military ones and the big ones. The big ones seem to be somewhat bugged, since they are almost impossbile to headshot

If a "doctor/nurse" zombie is hiding amongst many other ones, well then they are hard as well. When facing 3 or more joggers, issue can appear depending on weapon, though shotgun solves that issue I guess...
No IMHO, military zombies are NOT groundbreaking. And since you seldom meet really, really large groups of zombies well the game isnt that hard. Havent played last/final battle. So cant comment on that difficulty. With pistol and melee, all zombies are hard, the problem is that you get preety hardcore weapons early (if lucky). On my first playthrough and second mission I got the M16 (two of em), and also the M1 Garand as well the Combat shotgun with PLENTY of ammo. THAT was gamebreaking, since before that everything was VERY hard. Guess what protected those items? Two large football players in a "Two Floored Villa". It wasnt really that hard to reach those items since I also were lucky that no zombies were outside to bother me, when luring the two fellas out for a little football game...
EDIT: I must say that I still believe it was the right choice for Kerberos to allow "loot/missions" to be random, since it prolongs the game and it also feels less like your walking on a red line. I guess I were very lucky my first playthrough
