Trust is receptivity to enhancement. So if we think that a thing or person, for instance God, is 100% reliable, doesn't that mean we trust that thing or person 100%?
It might sound like we would. But part of trusting something is being awake to it. That's part of receptivity to enhancement. To even consider something as being enhancement is part of what makes it enhancement. On a deeper level, we can't receive the gift of a person unless we value them. So we can't trust them if we don't value them. And we tend to value the things that are contested. So then maybe the tails come apart on trust. An obvious form of trust is to find something safe, but another is to be awake to it, to connect to it, and the two values may exclude each other to some extent if either is maximized. Thus the way to increase trust to the maximum in something (especially obvious with a person) is to not take it for granted, and if we take "not take for granted" literally, we would need to not be assured of it 100%.
People used to care more about the assurance that they were saved, and that God existed. There's an extent to which anxiety over these things inhibits trust in God. So assurance is a good thing -- up to a point. >90% or even >99% assurance may be good, but 100% assurance of the existence of God and of one's salvation are things that aren't so clearly good. Using numbers here is a little misleading (are all 99% assurances the same?). So maybe the real point is to not take God for granted, or our own salvation.
In times past, people were more certain that God existed, and less sure that they were saved, but now it is more true that we think that if God exists, we're definitely saved, but God may not exist. In either case, we are kept out of complacency, which is at least partially a good thing. I think that maybe it's okay to be 100% sure that God exists, as long as you are not 100% sure that you are saved, and that's preferable to me than the other way around.
As to belief in God, not being 100% sure of the existence of God can keep you awake to the truth. You keep asking "does the truth say that God exists?" If God is truth, then truth should say that God exists, so you should listen to truth, be open to it on a deeper level. So then it could be good to not be 100% sure that God exists, so that truth can tell you that he does.
Lack of 100% certainty isn't a good in itself, rather trust is. Insofar as lack of 100% certainty awakens you and teaches you to value, it is a good thing. Uncertainty of a certain kind or degree can cause you to trust less.
The MSLN position on assurance is that until we are completely done with sin, there is some chance that we can get to a state where we hold onto a sin forever, never letting God work it out of us. In this life, we never know if we've reached that state of being done with sin, where no new sins can come to light, and we can never be sure that we are done being tempted, even if we get to some state where it seems like we aren't being tempted anymore. But when we are done with sin, in heaven, then can we have 100% assurance? Maybe, but why? We would have God himself instead.
I can picture a lot of people living in innocence, rather than presumption, when they take God for granted. Maybe children or child-like people can be forgiven their taking God for granted in that way. But children grow up, and maybe we will all grow up, in this life or in the Millennium, and have to grapple with the fact that we have sins in us, ways in which we are incompatible with God, and as long as we do, there is some risk, small though it may be, that we will commit to those sins more than to him and become hardened.
Also, I can see that people might say "the way to not take God for granted is to be grateful for him and his gifts, rather than to doubt your own salvation". One aspect of trusting someone is listening to them, waiting to hear what they have to say, and valuing what they have already said, responding to it. It's possible to be very grateful to someone and not listen to them. In this case, trust is defective. A person who is uncertain (but not perniciously uncertain) of another person will listen to what they say and take them seriously, and thus trust them. The stance of listening to God, listening to hear if we need to let go of any more idols or sins, is similar to or exactly the same as (a certain kind of) doubting your own salvation. There's nothing wrong with gratitude (certainly it's more trusting than being positively ungrateful), but it doesn't solve the problem of not taking God for granted by itself.