It's my policy not to pitch or work on spec.
I'd rather walk away as people can see what I do from my folio even though it's hard to do so sometimes especially when work is thin on the ground.
That being said, I once did but it was for one of those clients and a job that couldn't be refused.
It was through one of the worlds best known agencies for probably the biggest event (I'm still under an NDA).
It was to design mascots with variants the the main and partner event, show how it's be implemented for billboards and everything else.
I spent just under a week working all hours putting a presentation together.
I didn't get the gig but I was amongst some good company but I was actually quite relieved as it's considered to be a bit of a poison chalice.
Thing is, it's a massive investment to pitch for a project especially if it's a company that has to pay it's staff.
They pay twice. Once for the pitch and then the lost revenue.
You may end up doing more than half the work on a project to get it to a pitch stage.
Just recently I was approached to do a job and agreed.
I got the brief and it was obvious I was pitching against other Designer/Illustrators so I walked.
Say no to spec!