While I like the 5 members party for combat, due the limitation of the party size, people that beats the game just ones (I'm guessing is the most) miss out the companions that are not on their top 4 favorites (for this particular party size) This is how it works: your party can have 5 or less characters This is, roughly, the idea: you can have all the traveling companions with you, you can fight with 5 or less characters.
When combat starts you could ask when are your other companions, but is just as if you would ask why cant you bring more than 5 people with you every time you see your party walking, experience the reaction of some companion, or anything that can bring to your thinking the fact that you cant.
The mainly difference is that you accept it, are get used to it and don't think about it. Something like this would remove from out of the combat experience, the history, the limitation created to solve combat problems, that is why i mention it, and ask the developers to consider something like that. Maybe a simplistic optional mechanic.