I can think of a reason: taxes. Let's say you had a sizable balance of MR points. You need to make a business-related flight, and have determined that buying MR points and transferring them to an airline for an award ticket is much cheaper than buying the cheapest available revenue ticket. In that case, you'd want to buy MR points and do the transfer, so that you can claim a tax deduction for the MR points purchase as a business travel expense. Note that if you just transfer existing personal MR points for this purpose, you can't somehow get reimbursed for their "value" by the business for an equivalent amount of the MR purchase price, as accumulated MR points have no value as far as the IRS is concerned. Such a "reimbursement" would be considered taxable income by the IRS.

I suppose anyone with this particular problem could easily solve it by getting a business Amex card and setting up a separate MR account.