No limit on space available travel for spouses and kids. I think parents have unlimited domestic travel but only a certain number of international travel days they can use.

ID90s allow you and your family to travel on airlines besides your own or airlines that may not have jumpseat agreements. Great for international travel if you want to get some place Delta just doesn't go (Tokyo to Sydney, for example), if the wife and/or kids want to get on another airline besides Delta, or as a backup for a route that Delta may fly but other airlines serve as well (gives you the option of flying KLM if AMS-JFK on Delta fills up). If you have the option to ride your own metal then ID90s are NOT better than space available (SA). Your priority is lower (often at or near the bottom), the fees can add up, they tend to be less flexible and you are almost always not eligible for an upgrade (though you can sometimes get lucky). Hope this helps clear things up.