You can't accept invites or invite people while you are in the book waiting line... You have to stand around missing out on chances to actually progress
There is a difference between just giving people in groups more hallmarks(which they already largely get from doing harder difficulties) and having a whole new currency that soloers can't access... I am fine with groups getting more hallmarks and I supported the idea of adding something like meebles where you automatically get a bonus for having additional people. The difference is I can work hard solo to "catch up" on hallmarks but I can't solo a new "group only" currency. One of these situations punishes soloers - one of these simply rewards parties. I'd rather have the latter without the former.