I think this is way too easy to go infinite. Should probably have the same safety valve as Yawgmoth's Will: "If a creature you control would die this turn, exile it instead."

Exiling stuff brings it more in line with other effects/themes in the set too. MAKE IT SO!

Nice tension if you can't get rid of it afterwards, too.

Whenever I see "whenever" and instead" in the same ability, I can't enjoy the card and wonder "Was a triggered ability or a replacement ability meant here?" instead.

Considering the context of the first ability, you probably want to use "if" rather than "whenever" in the second ability.

No matter the finer detais of the wording: This comes with weird memory issues if you cast an unrelated creature spell or during the same turn a player had Spiritbinder ETB e. g. an end-of-turn Whisper Agent.

Even worse: Imagine playing Shambling Vent the turn you cast this and several turns later animating it. Now technically its a creature that entered the battlefield the same turn as Spiritbinder.

A third reason the wording of the ability is questionable: On a cursory glance it seems like "the same turn as Spiritbinder dies" is a continuous phrase and the verb refers to this permanent, but the rest of the sentence doesn't work out that way. People will be left baffled.

It should all be part of the same ETB trigger. "When ~ ETBs, you may cast ... If a creature you control would die this turn, ..."

There's no memory issues, because it exiles all creatures, not just ones cast using its ability. If you wanted to restrict it to just those (which you shouldn't), you could put a counter on them. "When ~ ETBs, you may cast ... Creature cards cast this way ETB with a risen counter on them. // If a creature you control with a risen counter on it would die, exile it instead."