[Alex Murphy]

Why would it be a “waste of time” to pray that someone who has committed suicide does not go to Hell?

It's only a waste of time (assuming suicide sends one to Hell) if they're already dead yet not already there. Is there a "purgatory" for the Hell-bound?

[RoadGumby]

Purgatory does not exist. The idea of Purgatory is only rooted in the idea that the blood of Jesus is INSUFFICIENT to bring you to Heaven. My belief and hope is in Christ, now and after death, not in the prayers of those I leave behind.

[SoothingDave]

It's only a waste of time (assuming suicide sends one to Hell) if they're already dead yet not already there.

If God is bound by time, it is pointless to ask Him for mercy for the dead.

However, if you believe God is not bound by time, then our prayers can be given for all: living, dead, not-yet-created.