You make it sound like Saber will run out of mana/prana. She's being properly supplied by Kiritsugu, it's not like the 5th War. Excalibur is draining, but it's not going to put her at a huge disadvantage. She just can't fire it in succession, plus it takes a while to charge.

It's like Lina Inverse's Dragon Slave from the movies (not the Slayers tv series where it is used comically). Actually, the special effects used in this episode make it appear very similar to the movie kind.