[quote=@Zobozun]
And?
Aren't servants sustainable for a while even without that?
Pretty sure Gil was going strong for years and years when we was with that priest.
[/quote]

Gil was materialized by the mud. Independent Action A+. He didn't even need orphan juice.

A normal Servant needs to burn prana to maintain the status effect of "no Gaia I'm alive really I am". 

That's why Rin has to burn the vast majority of her prana just to maintain Saber in Sunny Day, and why Illya was dying every time Herc moved before the Greater Grail activated. And those two are way beyond any Masters here.