I like when the arp jams up. I've had it on a MIDI track (one that sends MIDI to outboard gear) just continuously play despite hitting the "panic" button and/or turning on/off the audio engine.
The built in arp is useful however, and I use it often. I like using it to just continuously play a 1/4 or 1/8 note to an outboard synth as I build and tweak patches.