People don't feel the MSPE is that useful for two reasons:
1. It is just the sum of its parts. They already get your grades via your transcripts. They already get your USMLE scores. They already get 3-4 letters of recommendation. While the MSPE is nice in that it compiles all that information into one place and provides some comparative statistics, it really doesn't add that much to the other stuff we already have to evaluate applicants.
2. It used to come out in November. Some fields would specifically wait for this to send out interview invites, but for most fields they had already evaluated the applicants and sent out the vast majority of their interview slots before the MSPE was ever even released. So historically most program directors learned to get by without it. It's only in the last couple of years that it has come out earlier.
Now, I do think it has some uses. For one it is really the only place that red flags (professionalism issues, history of substance issues, suspensions, withdrawals, time off, e.g.) comes out and they usually provide some explanation of that. I also find it helpful to read the subjective comments from the clerkship grades (although some schools selectively edit these).
And simply it puts everything in one place which is nice. However, our program and I suspect many others go even a step further. Our PD's admin staff creates a one page summary for all
of the applicants with board scores, clerkship scores (with % cutoffs and NBME scores when available), class rank/MSPE adjective (with definition), # of pubs, etc.