It did indeed happen in history and Alfred was Guthrum's godfather. It was a widespread policy famously used by Charlemagne to bind new conquests to him. I think you are thinking too much of Christianity as a religion and so reducing its actual place in society. By baptising Viking leaders and crucially being their godfather Christian rulers are:

1. First and foremost accepting their submission. We have detailed description of Charlemagne's baptismal ceremonies in which the conquered rulers would be forced to kneel before Charlemagne and perform an act of submission in front of their followers, this could be a kiss of his hand or washing his feet. The power dynamic was clear this was very much a feudal show of fealty.

2. Welcome them into the Christian community. There is a reason Christianity is so widespread and Kings tended to be keen to convert their populations. It gave so much more power to monarchs, Viking leaders relied on reputation, gifts and victories to gain a following. They were limited in the resources they could muster or even be aware that they have. It was hoped Christianity would provide so many benefits with its army of educated clerks and sermons to the masses that the Viking rulers would change to more stable rulers. If you don't need to have a big reputation to stay in power there would be less desire to constantly look for new conquests and victories.

3. Christianity can be a 5th column for Alfred and the other Christian kings. Though our evidence of this is limited, The Last Kingdom (or at least the books) detail a network of intelligence coming to Alfred from monks and churches all over the Viking occupied lands.

4. Those lands were Christian prior to the Viking invasion. This gives Wessex the least material benefits of conversion but actually it probably was what Alfred felt the most important reason. The people overrun by Vikings were Christian before and many of these still lived in those lands albeit now dominated by Viking communities. There were still large clusters of Christian towns on Durham and Cumbria as well as smaller settlements. These Christians must have been constantly at risk of Viking persecution and eradication of their religion. Baptising Vikings is a way of removing religion from the equation and protecting churches and monestries in conquered Viking lands irregardless of how well behaved Viking leaders actually turned out to be.

It is also clear after baptising Guthrum, Alfred underwent a program of preparing for a future war. He wasn't foolish enough to think that Christianity was about to turn Guthrum into a saint.