Cables are supposed to be supported, so clips every so often will support it.
However, in practice, when most rewires are carried out it is often unpractical to lift too many floorboards, so often cables have few or no clips.
I wouldn't worry too much, but since you have easy access it may be worth doing to tidy things up for next time anyone has to go down there for maintenance. If you do go down there check for mice damage. And make sure the cables aren't touching the pipework.
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