I think the vast majority of people (and probably businesses as well) would just put 100% of the expense through, even if there is cash back on the card. Would you include the 2% as income? I don't know.

For starters, the recordkeeping would be a pain. If you had a tiered rate card, would you deduct 5% for gas, but 1% for the car rental for a trip you made for work? What if it was one of those cards which gave you 5% on the top categories of spending, would you wait until the statement came out, which might not necessarily coordinate with the deadlines for expense reports?

Second, the reimbursed amount per the expense report wouldn't agree to the receipts. Granted, they would be lower than what the receipt says, but it might raise some flags.

Of course, some companies have corporate cards, this being partially the reason why. But some companies might just consider it part of doing business (just like how for most people, they aren't obligated to use their company-earned miles to book future company travel).