It can, in theory. In practice, there are some ports that actually need a full Xcode installation to build, and those would just fail without Xcode.
In an ideal world, there would be a dependency declared on Xcode for all those ports so MacPorts would work with command line tools only and fail gracefully whenever Xcode is required. However, that is currently not the case because nobody implemented an easy and automated way to tell whether a port will build fine without Xcode.
Yes, that's true, and of course that's what I'd like to do (command line tools only), but last time through that was yet another ten minutes to find the correct xcode and match things up and so on ...
I fully accept that I'm just being a complainer here, but in answer to the parents question, those are the mental blocks for me.
In an ideal world, there would be a dependency declared on Xcode for all those ports so MacPorts would work with command line tools only and fail gracefully whenever Xcode is required. However, that is currently not the case because nobody implemented an easy and automated way to tell whether a port will build fine without Xcode.