I like them, particularly in the many MMOs were holiday events are the main form of content update now (Guild Wars, most SOE products, etc.).

They are almost always limited to 1-3 geographical hotspots anyway (since decorating or adding weather effects to entire games would be impractical), so I don't know how people can 'over-dose' on them anyway?

Choosing to go to the busiest community of a hotspots on the most festive days of the year then complaining about the abundance of people enjoying festivities there is kind of a sympathy non-starter; like going into a nightclub to complain about loud music on the radio, or a strip-bar to complain about T&A on television.