In some ways I think Apoth is less necessary than just someone trained in Medicae carrying a narthecium. That's usually an Apoth, but it doesn't have to be. Your Techmarine could be trained and have a Narthe; especially if you lack an Apoth I'd be willing to allow someone to Elite Advance Medicae (for double the regular cost probably) just to have that option on the table.
The sweetest part about Apoths is the +1d5 wounds healed special ability which, when combined with a high Intelligence, Narthecium, and Master Chirurgeon basically means an Apoth can heal a Marine from light crits back up to full wounds in one test.
So Apoths do heavily skew game balance both ways. I've run games with PC Apoths and one with an NPC, and every time they pretty much are the foundation of the KT - the PCs will take greater risks when they think healing is more likely.
Without one, you need to be careful of how hard the fights really are, because the only option to heal is with Fate (which can make things Easy Mode in its own right, honestly, depending on session pacing).
With one, you almost have to go more overboard because of how quick and easy healing is - so much so that I house-ruled it to take much longer than a Full-Round Action because it was impossible to keep damage on players between fights (I think I ended up at 5 minutes per person, which is still VERY quick by medical standards, but in a lot of situations the KT didn't have 20 minutes to spare). Increasing the time it took to be treated made them actually think tactically, versus assuming "we have 30 sec for the Apoth to patch us all back to full."
A note on Devastators: yes, they help chew through big hordes. But fighting huge hordes to begin with quickly turns into nothing more than a tedious affair of back-and-forth volleys and math, Devastator or no.
A note on Librarians: the DW phenomena rules are weak-sauce, especially Rite of Sanctioning (which I would ban, because it removes a lot of issues with psychic powers). I made manifesting a lot more dangerous (i.e. phenomena can happen a lot more often), which I feel like fits with the setting, and my Librarians have been pretty thrilled with it surprisingly.
Edited by Kshatriya, 25 April 2014 - 07:34 AM.