G-protein coupled receptors involve complex cascades mediated by cAMP second messengers and protein kinases. These receptors allow for delays and long term effects. End plate potentials involve extremely short term electrical interactions mediated by small changes in membrane potential. These dissipiate in far less than a second. Ligand-gated ion channels open rapidly, then have their ligand disssociate, then shut again. If the ligand binds irreversibly, you could have a long term effect until the receptor molecule is recycled, but few ligands bind irreversibly so it's not a major effect. At the neuromuscular junction, membrane potential is changed rapidly to accommodate movement and change muscle cell length. It's very unusual to maintain one position in voluntary muscles for extended periods.