1) Since design sits between art and engineering mock-ups should be backed by statistical analysis. If this is not the case it is not design, it’s painting.
If you analyze your numbers correctly it is very rare that they justify or suggest a second design.
2) The devil is in the details. The amount of detail that makes up a great design is huge. Every single element must designed and placed in its optimal position with the right style while keeping the whole balanced and aesthetically pleasing. Meanwhile it must be in line with business goals and be technically feasible. The designer must walk the fine line of compromises that lead to a good design each time reasoning against competing constraints. It is impossible to do this dance twice and still produce a different design (unless suffering from split personality disorders)
3) A good designer will always be opinionated. He/she will have an idea on what the end result should be based on instinct, experience and eye for aesthetics. This is why he gets the big money after all. Asking the same designer to make a second mock-up is like asking him to change his personality.
4) If the client really needs 2 mock-ups to decide then it’s fairly certain that the client does not know what he wants in the first place and any amount of mock-ups will not help. By sticking to one design and incrementally refining it will help both the designer and the client understand what needs to be done. It also helps keeping both parties focused instead of overloading them with too much choices and options. At the end of the day the client’s focus and feedback are important resources and it’s counterproductive to dilute them on multiple mock-ups.