It isn't extremely common to do so, but the most basic example I can give is incidentals at a hotel. The hotel room and some incidentals are reimbursed by the company, but other incidentals are not (ie some consulting clients do not allow the reimbursement of alcohol). It's typically easier to just put it all on the corporate card and mark some of the transactions as reimbursable and some of them as personal than it is to finagle a way to settle the hotel bill with multiple credit cards.

There's also the one-off situation where I really needed a new pair of pants while traveling (my only clean pair of business-appropriate pants split down the middle) and for some reason the only card I had on me was my corporate card. Obviously extremely rare, but I've heard of similar situation (but with a belt) happening to another colleague.