In-ear consumer earphones, designed to be used with personal stereos and mp3 players, vary greatly in quality from the cheapest, which are supplied in the package with audio players, to expensive alternatives which sometimes offer excellent audio quality. As such, their use is by no means to be rejected out of hand.
However, their open back design and loose fitting usually mean that consumer earphones do not reject ambient noise to the same degree as professional in-ear systems: the very feature that makes them safe to wear when crossing the road is usually a disadvantage on stage. However, for quieter gigs they may be perfect, providing a mix between ambient sound and monitor signal without resorting to the use of an ambient mic. On louder gigs, however, it is possible that the only way to achieve sufficient volume above the ambient noise level with consumer headphones is to turn them up so loud that both distortion and hearing damage may result.
An interesting hybrid solution involves the use of sealed, custom moulded 'ear adapters' which attach to standard sized consumer earphones.