Here is the entry for magic weapon on SRD, and here is greater magic weapon, for reference.
To summarize, magic weapon...
- is a level 1 spell for everyone who gets it,
- requires only a holy symbol as a component,
- confers a flat +1 bonus to the weapon's hit and dam rolls,
- and lasts for one minute per level.
- is a level 3 spell (4 for clerics),
- requires material components,
- confers up to a +5 bonus on the weapon's hit and dam rolls,
- and lasts for one hour per level.
This spell could see more use in the suggested (SRD) form, especially given the weapons out there of special material or exotic type that do not, or extremely rarely, come enchanted. Paladins and fighter-friends of wizards and clerics could benefit by having, say, their cold iron weapon be made magical for the upcoming demon fight. Even low-level priests and especially wizards could gain a little bit of a bump given their dependancy on weapons and the relative rarity of magical staves and daggers. It is almost never used as is, likely, because of too-high limitations for too-little gain.
Thanks for reading.