Its just not the greatest mixer. It colours the sound, especially when you run stereo on one channel.
No dude, it just has a 3db panning law. This means that a stereo channel will be 3db less in volume when it's centred than when it's panned hard left or right. If you pan, this is a good thing - it stops sounds from disappearing when panned. It also offers headroom for when you're actually mixing, rather than when you're just tossing off over a single patch.

That's the reason why panning hard into two mono tracks seems to sound higher quality. 3db is too little a difference in volume for our feeble human brains to detect it as a difference in level (the average brain requires at least a 6db difference before it can consistently detect that the volume's different); instead, it's detected as a difference in quality because THINGS AWAYS SOUND BETTER TURNED UP.

So no, there's nothing wrong with Reason's mixer, despite internet rumour to the contrary, and if you don't like the panning law you can always use the trick you described above (but don't complain when you run out of headroom when you actually *gasp* try to mix it with something). :)