only problem i've had with leaving them on is after off roading the ARB can get stuck with one side up an the other down, an it handles like ****e when it happens an scrubs ya tyres and not to mention puts the disco on the pi#s
read this thread, it took me ages to work out what was happening and it was only when i posted piccys of it on here that some nice person pointed it out, just so you know what to keep an eye on when you've been off road
http://www.landyzone.co.uk/lz/f8/discos-pi-s-help-68695.html
oh btw last time i went off road BOTH ball joints went to the top of the axle.