I have seen people destroy nylon string classical guitars doing this.

Unlike steel string acoustic guitars the bridge on most nylon string guitar's is only held on with glue. If you put steel acoustic guitar strings on this, well... first the back of the bridge raises and this get progressively worse over time.

Not only does this screw up the tuning and intonation it also leads to the guitar needing the bridge re-glewing or in some cases throwing away.

Im not taking the piss or anything but Id be really interested to hear a sound clip of the nylon string guitars strung up with 9s. If you could post one here, that would be great.