[G6Tanuki]

"Buying another tube to keep as a spare" always assumes you have somewhere safe to store it. Keeping a spare fluorescent tube in the cupboard-under-the-stairs or similar probably means it will end up being broken before it's needed. Having a spare also somehow mystically ensures that the one in-service will now last forever so your spare is never needed.

Also consider the 'opportunity cost' of your spare - what else could you better have used that money for rather than tying it up in a spare?

My parents were in the habit of doing this 'buy one extra as a spare' thing: when they died and we did the house-clearance all these largely-unidentified 'spares' cluttering the utility-room and attic went in the skip, because the things they were the spares-for had long since been disposed-of anyway.

I really must work out how to dispose of the unused CFL bulbs I have here - a 'gift' from British Gas a couple of decades back, when they were on one of their energy-efficiency drives. I dread to think how much this dented their profits. Charity-shops don't want 'em.