ETA's were never required in a CTAF inbound call.
The only required information is callsign, aircraft type, position, level and intentions. (AIP ENR 1.1-79 p.63.1 or JEPP AU-718 p.6.5.1)
The problem is, most instructors are just following the incorrect radio calls taught to them by their instructor and the issue is passed on if the student never bothers to read the AIP/JEPPS. e.g. People reporting "traffic sighted" in a GAAP and reading back sequencing numbers...WRONG!!
In any case, do you really think an ETA to the minute has that much bearing on when you start to look for traffic? Remember that ETA is their circuit time and you should be looking for them well before they join anyway. If people give their call at the correct time then it's not going to be any more than a few minutes before you start to look.