core.js in sharepoint master pages

in sharepoint default.aspx file, core.js is loaded as deferred:
<sharepoint:scriptlink language="javascript" name="core.js" Defer="true" runat="server" / >

in layouts/application.aspx, it is loaded without deferred:
<sharepoint:scriptlink language="javascript" name="core.js" runat="server" >

The impact of this difference is, you can override javascript functions in application pages (layouts pages), but you can't do that to site pages.