It’s quite easy to accidentally convert a live dimension object into a dead group (reported here: http://illustrator.uservoice.com/forums/601447/suggestions/47739104)

When it happens, there is no way to tell if a dimension is live or dead but to try and select it, or view in Layers, selectin them one by one.

What we need (even if the bug above get fixed) is an ability to select all the dimensions at once.

The powerful and free Select Menu plugin (https://rj-graffix.com/downloads/plugins/#selectmenu) doesn’t have this item yet as well.