I've recently done 11 hours and 13 hours with BA and three 4/5 hour sectors with LAN, all in Business. The crews weren't visible once food had been served so my first reaction is to agree with you. On the other hand, I slept most of the way through the 11 hour overnight and a lot of the way through the 13 hour overnight so I don't actually know whether they were visible or not. I was quite happy with the LAN flights but the crew (lack of) visibility was more obvious on daylight flights. Except one; the last of our three flights we had a male crew member looking after us and, compared to the other crews, he was a revalation (and I've written to LAN to tell them so). He introduced himself, was very industrious during boarding and was generally visible.
It's a balancing act. I did notice one crew member walk quietly through the cabin during a wakeful period on the 13 hour flight and we had bottled water on both flights. BA also have their "raid the larder" so you wander off and nibble/drink as you like. When I'm asleep the less movement the better, probably. Even on a daylight flight how often do I want someone coming through to check on me? I'm not sure I know.