原文來自這里。
今天用Xcode5的時候，發現默認的IBoutlet的屬性設置為weak——因為Xcode5建立的工程都是ARC的了。但是當時還有點不明白，因為項目的原因，一直沒有正式使用過ARC。於是，為了搞清楚為什么，google了一下，有很多答案。試着從Apple文檔尋找線索，在這里找到了說明：
From a practical perspective, in iOS and OS X outlets should be defined as declared properties. Outlets should generally be
weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should bestrong. Outlets that you create should therefore typically beweak, because:
- Outlets that you create to subviews of a view controller’s view or a window controller’s window, for example, are arbitrary references between objects that do not imply ownership.
- The strong outlets are frequently specified by framework classes (for example,
UIViewController’sviewoutlet, orNSWindowController’swindowoutlet).
簡單的說，如果IBOutlet對象是nib/sb scene的擁有者（File’s owner）所持有的對象，那么很顯然擁有者必須“擁有”對象的指針，因此屬性應設置為strong。而其他的IBOutlet對象的屬性需要設置為weak，因為擁有者並不需要“擁有”他們的指針。舉例來說，UIViewController的view屬性是strong，因為controller要直接擁有view。而添加到view上的subviews，作為IBOutlet只需要設置為weak就可以了，因為他們不是controller直接擁有的。直接擁有subviews的是controller的view，ARC會幫助管理內存。
緊接着，文檔里又提到：
Outlets should be changed to
strongwhen the outlet should be considered to own the referenced object:
- As indicated previously, this is often the case with File’s Owner—top level objects in a nib file are frequently considered to be owned by the File’s Owner.
- You may in some situations need an object from a nib file to exist outside of its original container. For example, you might have an outlet for a view that can be temporarily removed from its initial view hierarchy and must therefore be maintained independently.
第一種情形前面已經解釋過了，對於第二種，通俗點將，就是controller需要直接控制某一個subview並且將subview添加到其他的view tree上去。
單純從ARC的角度思考，用weak也是很顯然的：因為subview添加到view上時，view會“擁有”subview。當然，給IBOutlet屬性設置為strong也沒有錯，“糾結誰對誰錯“的問題可能需要上升到模式或者編碼習慣的問題，已經超出本文的范圍。
最后附上stakoverflow上的答案：
http://stackoverflow.com/questions/7678469/should-iboutlets-be-strong-or-weak-under-arc/7729141#7729141
除了選中的答案之外，其他人的回答也非常有意思，大家可以看看。