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Jolene said..
If the mast pumps in heavy seas or correct forestay tension is hard to achieve, then you need runners or checkstays.
I think that any fractional rigged boat or any boat with an inner staysail (cutter) that is going blue water cruising should have them. They are cheap and effective and can be easy to remove when not necessary
Sounds reasonable, a bit cumbersome in restricted waters short handed. This is why the question of necessity.eg Sydney harbour.