I try to always look at it as if I were in his position. For example, would I be comfortable with my boyfriend having a female 'just a friend' that he hangs out with all the time and watches movies with when I'm not around? Absolutely not. I admit, I can be on the jealous side, but it also has to do with me trying to be open minded in the past and let it slide in former relationships, only to get burned in the end and find out my boyfriend was the town bicycle.
I personally see it as unnecessary stress on the relationship. You can trust someone completely, but if you choose to ignore warning signs when they present themselves, it borders on naivety. Having these 'just friends' kicking around is a red flag in any relationship. YOU may know that it's innocent, and it may truly be, but he can only take your word for it. So where does he (or anyone for that matter) draw the line between "I really want to trust them..." and ending up feeling like an ass after they've been cheated on?
I'm not saying that you would do that, of course, I'm just saying that trust is a very fragile thing, and I personally would never compromise the trust of my partner for movie time with a friend :-/