Yes, JOSM does have a search function. But it lacks a replace function. If you have a data set containing 1000+ ways whose names include abbreviations (such as any US county initially mapped by TIGER as I'm facing here in Marion County, Oregon), it doesn't actually save you any time. Being able to replace substrings of key values (like bulk changing " Rd " to " Road " or " Blvd " to " Boulevard " in name keys) would be extremely useful, at least to US mappers cleaning up TIGER data.

Plus, there's a lot of data in keys that aren't necessarily a name (note and comment keys come to mind forthright) where a spell checker would be especially helpful. Perhaps a speller implementation would, by default, not check name keys on ways with highway=* tags, and when enabled, warns that blindly trusting the spell checker on street names is probably a bad idea unless you have a dictionary containing the names already. This would be useful for anybody who is training (my|a|i)spell with place names, and useful for catching typos in place names for people who already have place names in their dictionaries.