Clarification needed for comment in storage/file/fd.c

Hi, 
     I came across this comment in storage/file/fd.c while I was trying to write an FDW.

" AllocateFile, AllocateDir, OpenPipeStream and OpenTransientFile arewrappers around fopen(3), opendir(3), popen(3) and open(2), respectively.They behave like the corresponding native functions, except that the handleis registered with the current subtransaction, and will be automaticallyclosed at abort. These are intended mainly for short operations likereading a configuration file; there is a limit on the number of files thatcan be opened using these functions at any one time. "
It states that it behaves as native functions with the handle registered to the current subtransaction. 
If it is so, why is it intended for short operations?
Will it work if I open a file at the start of the transaction and close it at the end of the transaction?
What are the side effects/limitations of these functions?

Regards,
Samuel CC