What??!!!! Jim, do you know if that was before Premiere became Premiere Pro, or after? I wonder why anyone would have ever asked for that in a FR since it only imposes limitations and no clear benefits. If you ingested footage from tapes as single 1hr clips, then it might be of benefit (so you don't trim past the beginning or end of a take by mistake), but Premiere allows users to automatically break up footage from tape into individual clips, so then that benefit is lost.
Perhaps a Preferences Window checkbox is in order then, so users can choose: 'Timeline trimable subclips' (checked on by default). This way Adobe wouldn't lose the extra work they spent on the 'Edit Subclips...' dialogue box to serve the needs of whoever requested this for whichever reason.