there are many reasons NOT to investigate shrub and the gang for wrongdoings in office.
none of them are good.
one reason is that it sets a precedent for investigating the previous presidency, which obama would not especially care to invite. i'm sure he believes that his administration will have nothing to hide, but no one like to invite an investigation, at best their a huge waste of time and money.
another reason is that it (partially) takes the media spotlight away from obama and his new ideas for the present and the future. so it's an unproductive political distraction. well, surely there's enough media bombardment to cover more than one story at a time. besides, the investigation will be slow-paced and leave plenty of gaps for other stories. moreover, constant terrible news about the republicans is quite likely to work in obama's favor in terms of getting things done. especially because there's no political cost in the sense that you don't need to directly say something negative about the current batch of republican congresscritters whose support you might need. having the media bash the republicans constantly on other matters puts the current republicans in a spot where they really can't oppose obama and the democrats easily.
then of course, there's the possibility that any prosecution will go nowhere because not enough people will talk. they are a loyal, tight-knit bunch, after all. i think that's wrong, they only seem tight-knit because no pressure has yet been applied. some have already talked just for money. others will surely talk at the risk of prison time.
obama might not like it, but the country needs it. we CAN NOT PERMIT the next republican administration to play by the same "rules" shrub and the gang have played by. if nothing else, failure to prosecute means that obama's great works to come will endure only until the next republican administration.