Board Thread:Questions and Answers/@comment-11935184-20171009193837/@comment-294304-20171012161005

Saber can't go into spiritual form not because she's alive in Avalon...but because from the point she is summoned, she hasn't "died" or even reached Avalon yet. This is also why she only has Excalibur as a Noble Phantasm instead of Caliburn, Avalon or Rhongomyniad, all of which she had lost by that point. In other words, unlike the other Servants who are summoned in spirit form and materialized as familiars, Saber is summoned "alive".

I thiiink Zero Caster was able to go into spiritual form, though I admit it's been a while since I watched Zero. I don't think there is anything stopping them? (I recall it being actually easier on the mana drain than being materialized all the time)