Voted essential and have got more to add to the subject.
There is no need for the 'custom' text option.
We should be able to edit the dimension text itself by simply being able to write in the measured value box (although being able to edit the dimension text without a 'setting' box needing to be opened would be nice, as per the post http://archicad-talk.graphisoft.com/viewtopic.php?t=5692
I previously used Drafix (2D only and cheap as chips, but more than adequate for documenting residential houses - as long as you can think in 3D while drawing in 2D)
Drafix uses the <> symbols in the dimension text to represent the 'measured value'.
After the <> symbols you can type in any custom text you like.
As you stretched the dimension the measured value changed.
If you delete the <> symbols then the dimension displayed only the text you had typed in.
If you had a dimension where the measured value symbols had been deleted, simply typing in <> at the start of the text reinserted the measured value.
A wonderfully simple way of managing the dimensions measured value and custom text.
Also, with ArchiCAD's dimensioning, the revert to default position should be on by default so the dimesion text is always centered within it's length.