It is conceivable that somebody might want to transclude a page containing a LiquidThreads discussion. This leads to two different discussions: One on the subpage and one on the transcluding page.
This is not likely to be what the user expects and can cause confusion. On the other hand it seems convenient to turn a complex talk page into multiple groups of discussions using subpages. Activating LiquidThreads is then another step that is bound to be reinvened frequently (an anti-pattern one could say).
An example for such a use (but not currently containing any LiquidThreads discussions) is: http://strategy.wikimedia.org/wiki/Talk:Emerging_strategic_priorities/ESP_1_key_questions
You may be subject to mental influences from the http://en.wikiversity.org/wiki/Topic:Post-information_society