No Logic actually.
Attach Spare coaches to weekly trains while these coaches are attached to Daily trains where there is no such compulsion.
1)Like 22854 (SL/AC)coaches may be attached to 12572 SNSI express on Thursdays.(I have seen this Shunting twice).
2)I have also seen the shunting of 12572 SSPN Empty rake for 12834 ADI Exp.
3)Also...
more... I have seen Shunting taking place for 18006 Sambaleshwari express for 12571/12574.
The above scenarios have been observed by me more than 8-9 months back.
A)Generally The rake of 12837,12810 and 12863 are NOT Loose Shunted.