Janary transfers can be great deals that push teams on to reach their targets. The great success stories of recent years includes... Well, not many actually. Cisse, Suarez to some extent I suppose, I'm sure there are others, but I can't think of any right now.
There are plenty of examples of signings that have done very little to help the club that season and nowhere near were value for money. Torres and Carroll are the obvious recent ones, but even Evra and Vidic when they first signed for United in a January window were fairly poor for the rest of that season.
There is no reason to buy just to buy, I agree with you. If we can find the right player then we should and will go for it. Someone like Moutinho who knows AVB and AVB has worked with before sounds perfect, but honestly if we want someone to perform straight away we should primarily be looking at PL proven players. A new signing who doesn't work out straight away can also cause some harm, unrest in the squad and of course if that player gets to play ahead of someone who was actually performing earlier in the season that can affect the results directly.
That being said, let's sign Moutinho, Llorente and Willian please