However, there are a couple planned features that'll change this. Virtual collections is one, but the other is the $ operator in field selection..which I believe is planned for 2.1.
Even as-is though, they are useful. The tag example is simplistic...let me give you a real case from mogade.com. We have a scores collection, which looks something like:
{
_id: ObjectId('...'),
leaderboard: ObjectId('...'),
user: 'leto',
daily: {
points: 100,
data: 'level 10',
date: ....,
},
weekly: {
points: 150,
data: 'level 8',
date: ....,
},
overall: {
points: 300,
data: 'level 15',
date: ....,
}
}
Also, I wrote about collections vs embedded documents: http://mongly.com/Multiple-Collections-Versus-Embedded-Docum...
Edit: I wouldn't rely on the age of a feature request as a sign that there's something fundamentally wrong with a design. Not everything can be top priority.
It might not be exactly what you were looking for, but it could have the same outcome.