It's not so much that they can't be used 24/7, but that Li-Ion batteries really don't like staying at 100% charge (nor 0% either for that matter).
If you can configure your laptop to only charge the battery to ~80% or so, you'll extend the battery life significantly. The optimal range for long-term storage is ~40-60% charge.
If you can configure your laptop to only charge the battery to ~80% or so, you'll extend the battery life significantly. The optimal range for long-term storage is ~40-60% charge.