Swapped cassettes yesterday on my road bike and wondered to myself if there was any reason to keep the old one. I'm not a pack rat so I don't usually try to clutter up my bench any more than I have to.

Is there any good reason to keep the old one around? I can see hanging on to the spacers and lock ring, I guess but that's about it.

Generally no. You might want to save a spacer or two in case you ever lose one, but the sprockets are probably useless, even the ones in great shape, because for most people the same ones wear out first anyway. Otherwise you might clean it up and keep it as a paperweight.

I keep them as I do lots of single speed conversions. I use the spacers to fill the gap on the freehub body, and if I'm doing an SS conversion on a bike with vertical dropouts it helps to have a cog in every tooth count (15,16,17,18,19,20, etc) to find out which one will work and be the "magic ratio" for proper chain tension.

You can tear them apart and use the cogs for decorations too. I've seen some nice wind chimes made from old bicycle parts.

I have a hub with a cassette on it and I put it on my bike when I clean my chain with one of those cleaner dealies. Keeps the gunk from dripping on my rim and tires.

I did the same thing with an old freewheel hub. It holds the chain in a good position for cleaning without the danger of getting solvent and gunk on my good wheel.

FastJake's comment about decorations is also useful. My wife has a friend who makes jewelery out of old cogs and chain links.

There's some of us who like to make custom cassettes, so we gobble up old ones and separate them for the elusive cog we don't have.

Unfortunately many of those elusive cogs, particularly the middle and smaller ones in the 14-18T range have been badly worn by the original user and will not work with a new chain. If you ever find a source for new 16T 9 or 10-speed individual cogs, let me know!

You could always pass it on to someone with more tolerance for sub-optimal shifting.

It makes me sad when I read about people replacing their cassettes every time they replace their chains.

Ninja throwing stars, FTW!

Most are just junk but if one is slightly nicer with a few good cogs I may keep it to build up a single or five speed for a flipper. Old cogs at the price of free make great spacers.

Well, I get 7000 to 8500 miles out of a Shimano 105 or Campy Veloce 10-speed cassette and a Shimano or Wippermann 10-speed mid-level chain. I spend about $40-$45 each for the cassettes and chains. I suppose I could discard the chains at, say, 2000 mile intervals and make the cassette last maybe twice as long but spending $120 on extra chains to save a $45 cassette doesn't make sense to me.

I might think differently if I were using $250 Dura Ace or $350 Record cassettes.

It's a toss up which is the best way to squeeze maximum miles per buck out of chains and cassettes. Many old timers ride chains and cassettes together until they just won't run any more. Typically this is somewhere between 2% and 3% stretch, after which both have to be replaced, since the cassette is too shot to run with a new chain. One drawback of this method is it can be hard on chainrings and you might find yourself replacing everything.

Most people today replace chains nearer 1/2% stretch to save the cassette so it'll run with a new chain. Usually you can run 3-5 chains on a cassette this way, and run the last chain beyond 1/2% stretch since the cassette is shot anyway.

Which is smarter depends on the relative cost of chains and cassettes, but as hillrider points out using 4 chains to get the same life as one barely makes sense unless the cassette does outlast 4 chains, and the savings on chainrings offsets the added chain cost.

There's a third way, which many find to yield the most miles per drivetrain dollar. That's to rotate 2-4 chains at regular intervals, running the entire package until it dies. I manage my bikes this way, and can get 15-25,000 miles out of the package, and sometimes more.
There's no one answer for everyone, and variables like what percentage of the total mileage is on only a few sprockets, can alter the economics. But just like chain lube, everybody figures his way is the smartest.

I keep the spacers, they come in real handy.

Prevent someone from making some tacky Art project out of them

They make great candle holders!

Do you have a trainer? Could use the cassette on a spare trainer wheel if you don't already have one.

Didn't think of that for some reason. That would give me a reason to buy a dedicated trainer wheel.

That nicely illustrates fietsbob's comment about tacky art projects!