I wonder why a length specifier is used for the items.lastvalue column in PostgreSQL installations.

There are several custom applications thinkable which only process the last value of items. What could be more reasonable than to use items.lastvalue without having the need of querying history tables.
But caused by the current limitation to 255 characters data might be broken or inconsistent. Think about the item system.sw.packages what might be used for querying installed software. Or a tool that queries all 'vfs.file.contents["/etc/passwd"]' / 'vfs.file.contents["/etc/group"]' items and lists what users exists where or which numeric id is used where and for what, etc.
Items like system.sw.packages might often find usage without writing to history tables. In such cases there is no way to get a complete value at all.