We used DL99 all the time at LCY. Usually because Codeco wouldn't accept more than 2 delay codes so we would enter say 93 and 99 and then split 99 in the SI field.
we use Lufty's TANGO system and for some reason it only accepts 2 delay codes (even though it has boxes for EDL's) so we usually need to use 99 and an SI comment if 3 or more delays.
The reason all these systems only accept two delay codes is that there are not supposed to be more than two delay codes per flight (see my reply 11)