Vicicicca(doubt) is one of five mental factors that hinders progress in meditation.
Reserving judgment might sound like a neat trick. But Vicicicca is already there and your mind is already corrupted to start with. So the natural tendency of the mind is to lean towards doubt. Because most of your thoughts are tainted with ignorance. Accepting Nibbana with faith shuts off one major avenue for Vicicicca.
It would be hard to keep to any path without faith, unless you know by direct experience that the path is right and it leads to the intended destination. It's the same with Nibbana. If you have no faith in the path or the destination, you wouldn't even bother to follow it. Even if you do, the slightest setback can make you give up on it. But if you have faith, you'd be more inclined to look at your own shortcomings instead of doubting the path.
How many times we put our faith in regular people on a daily basis? When a doctor gives you medicine, even when your life's dependant on it, you don't do any in depth analysis of the drug before using it. When you buy food, you don't lab test it 1st to see if it's completely safe. You just go by the reputation and put your faith on the quality assurance certifications. You don't know any of those by direct knowledge. When you fly, you put your life in the hands of the pilot and the airliner. Will you still take the flight, if you can't make up your mind about the safety of the plane? :)
Compared to all that, having faith in Nibbana shouldn't be a hard thing to do. And it is also the wise thing to do since it keeps doubt at bay and prevents you from getting discouraged easily. It is the same case with believing in rebirth. For those who have trouble believing in that, the natural tendency of the mind is to lean towards nihilism. That is harmful for the progress in the path.