People who know how to use spanners don't let them slip... newbs (and I mean no disrespect) with spanners cause damage and a box end is less likely to round things off or come loose.
I use a 6 inch Snap On Blue Point English wrench (adjustable) I got from my dad for doing on the road adjustments... it is also thin enough to deal with any pedal.
I use wrench and spanner depending on who I am talking with... most folks here don't know what a spanner is until they hang around me at the shop for a time.