I'm assuming you mean use their rental allowance to pay off their mortgage, which is good and all, until

1) They lose their job and have no way to pay that mortgage

2) The value of the house drops precipitously from its purchase price (which it will, guaranteed), and said expat wants to move back to the UK, US, or wherever (or just loses their job). That expat then is saddled with dealing with the negative equity relating to their mortgage, basically, if they sell the house, they'll continue to pay off the mortgage difference between the mortgage (theirs) and the homes sale price, basically paying for nothing, and getting nothing in return.