This is a great topic, mostly because answers to it will illuminate critical processes around database design and development.
You could actually ask an inverse question: how many of you take the time to back up your databases? Nearly 100%.
So how could so many companies pay so little attention to the core of their entire dev efforts? At many of the firms where I've worked, an initial ERD might be created but from that point on, rarely if ever is it updated.
I come from the opposite direction. I always create an ERD to start my design/dev work, and always keep it updated by first revising the ERD and using the modeling tool (ErWin, Xcase, ERStudio) to generate the DDL, etc. It takes extra discipline to always do it this way, since when there's a crunch, the tendency is to go straight for the end zone...but there's always a future cost.
For me, since I work on complex models, it is far easier for me to look at one of my diagrams for table structures/relationships instead of walking down a long list of table names in SSMS.