I just finished reading the referenced link: http://elinux.org/images/4/45/Ipl_and_ubi.pdf
Please read the pdf file and you will find answers to these questions as presented by the author.
A user would still need to develop a IPL integrated Linux kernel. Initialization time would be the advantage to use this technique. Plus adding a wide variety of boot media at IPL. Time for this technique is shown as 820mS and I would like to see numbers for other media since the IPL kernel would need to identify and load drivers for that particular hardware along with other protocol overhead.