It's a bad argument because it gives the impression that MPs are employing family members and friends so that they can effectively earn double salary. No doubt, some of them do precisely this.
Secondly, it is nepotism; when a family member is employed, they are denying the job of someone else from the public. You could argue that a family member may be 'more efficient', but you're not exactly going to get MPs hiring people on an equal basis when there is the choice between hiring a family member, who will add £35 K to the family income, or a member of the public, who won't.
Thirdly, if you can find a member of the public who is better trained than a family member at this sort of job, then there is a serious problem with your recruitment process.
Finally, at a time when trust in politicians is (rightfully) at an all-time low, MPs should be doing everything they can to alter the impression that they're corrupt bastards in it for themselves and their rich buddies. They're playing with fire, here.