Many battles have occurred when one unit meets another and begins a skirmish, drawing the rest of the armies into a huge battle. You only have to look at the Battle of Gettysburg to realize this. So saying that they have to line up and face the enemy before engaging in battle is ludicrous. It may be preferred to line up before battle, but rarely do things go the way you want them to in war.
Yeah, cause Gettsburg was SUCH A GREAT medieval battle that required disciplined melee formations. We are talking an age were battle still often had agreed starting time so both armies could deploy. If you are going to ambush an enemy, then yes you are going to want to ensure your army has time to draw up into correct formations. Heavy Cav for example loses almost ALL of its effectiveness if it is not correct formed up. Depending on how soon they expected to meet the enemy we also need to factor in that units may not already be wearing full armour and have their battle loads equipped.
Sure you could use your skirmishing archers and MI to pester your opponent while you form up, and often did, but that doesn't detract from the fact that the way medieval armies marched didn't lend themselves to rapid deployment into fully equipped ready to go units.
Conversely the enemy would use THEIR skirmishing troops to do the same so they could form up into battle formations, or to cover them while they move to more advantageousness grounds, or they attempt to withdraw completely.
Notice I never said battles COULDN'T occur when two travelling armies meet, simply that it is not a given. Even if your scouts detect the enemy chances are you can't force them to battle, or you misjudge their travel direction and they slip by.
