Of the 5,364 items in my iTunes music library, 143 are songs I purchased on Apple’s (AAPL) iTunes store. The vast majority were ripped from my old CD collection. A few were obtained by other means.

My children have even larger digital music collections that they store on iTunes and play on their Apple devices. I venture to guess that like most kids their age, the vast majority of their music came, directly or via friends, from pirate sites. When I offer to gift them iTunes music, they tell me thanks, but no thanks. It’s not worth the hassle.

As I understand it, the iCloud service Steve Jobs introduced Monday — especially the feature called iTunes Match — is a bid to change that dynamic.

The feature that got top billing — Apple calls it iTunes in the Cloud — will have only modest impact, at least at first, because it only works with songs purchased through iTunes. If you sign up for iCloud (and it’s free, so a lot of people will) all your songs will be downloaded automatically to all your devices — as long as those devices were made by Apple or run Microsoft (MSFT) Vista or Windows 7.

For the rest of the music in your iTunes library — in my case, roughly 5,200 songs — Apple offers iTunes Match. Here’s how Apple describes it:

If you want all the benefits of iTunes in the Cloud for music you haven’t purchased from iTunes, iTunes Match is the perfect solution. It lets you store your entire collection, including music you’ve ripped from CDs or purchased somewhere other than iTunes. For just $24.99 a year.²

Here’s how it works: iTunes determines which songs in your collection are available in the iTunes Store. Any music with a match is automatically added to your iCloud library for you to listen to anytime, on any device. Since there are more than 18 million songs in the iTunes Store, most of your music is probably already in iCloud. All you have to upload is what iTunes can’t match. Which is much faster than starting from scratch. And all the music iTunes matches plays back at 256-Kbps iTunes Plus quality — even if your original copy was of lower quality.

That footnote after “$24.99 a year,” by the way, lists the devices iTunes Match works on and mentions a 25,000-song limit.

Here’s my question: Once Apple has replaced all the 5,000 plus non-iTunes songs in my music library with clean 256-Kbps non-DRM copies that are mine, permanently, with all the benefits of iTunes in the Cloud, why would I pay for a second year of the service? The job is done, thank you very much, I’ll take it from here.

For my kids — and all those other kids who are still building their music libraries — the question is more complicated. A one-time charge of $25 to convert up to 25,000 pirated songs to legal iTunes-plus quality copies is a no brainer. If they plan to continue stealing music, however, they’ll have to make a calculation at the end of the year. Have they collected enough new music to justify spending another $25 to bring them into the iTunes fold?

A few of them might even be tempted to buy a song or two on iTunes.

iTunes in the Cloud is available now in beta by downloading iTunes 10.3. iTunes Match is scheduled to arrive this fall.