Previously, an invitation to a private blog involved an email message with several options. Besides the expected invitation to join (using a Blogger / Google account), one might find an option to "preview" the blog, and / or view as a "guest" (without requiring a Blogger / Google account).
Both the "guest" and "preview" options caused confusion, for both owners and prospective members.
We might see any of several problem reports, in Blogger Help Forum: Get Help with an Issue?.
- Some would be readers would be confused why their membership had stopped working.
- Other readers would wonder why they could not comment - even with other members commenting.
- Owners would be concerned by complaints from their members, about unexplained membership loss.
- Some owners would be unable to re invite members - even with the "Blog Readers" list populated by less than 100 entries.
- Some owners would be concerned by members unable to comment.
Invited members using "guest" / "preview" would find access was temporary.
All invited members might not observe / understand the difference between the "accept membership" and the "view as guest / in preview" mode. When the temporary membership expired, some former "members" would find themselves unable to access the blog.
Invited members using "guest" / "preview" mode could not comment.
Other "members", who were not actually authenticated, would find themselves unable to comment - even when able to read the blog. This discovery might cause bad reputation for both commenting, and private blogs.
Blog owners could not always re add members, once access expired.
Blog owners might receive complaints from "members" unable to access a blog. In some cases, having invited more members after the former "member" had "accepted" membership, trying to re invite the former member would be unsuccessful, because the membership invitation limit had been reached.
Blog owners might find problems with membership invitation limits, in general.
Some blog owners might intentionally use the "guest" membership as an option to provide access to more than 100 viewers - but be unable to sustain the desired "membership" level, because Blogger had changed limit enforcement policies.
Blog owners would receive complaints from members unable to comment.
Other blog owners might use "guest" membership to provide access for readers who refuse to use a Blogger / Google account - but be unable to provide ability for "guests" to comment.
All of these issues created some bad reputation for private blog membership, in general - and some unnecessary problem reports involving commenting. With Blogger having removed the unreliable guest / preview modes, some blog owners may find themselves with less ability to provide blog access to large crowds of members.
In the long run, this will be a positive move.
People who wish to distribute private information, to large groups of people, will have to use other products. Google+ has been recommended, for several years, as a private information distribution medium.
Simplified options leads to simpler code - and to a more stable product.
Blogger design simply does not support large reader communities, of content requiring restricted access. Cleanup of this option, misused by a minority, will allow Blogger to simplify their code base - and provide a better supported product, to the majority of blog owners and readers.
An owner of a private blog, with more appropriate member population, will simply have to convince all readers to accept a Blogger account, as part of accessing the blog.
A Blogger account does not have to be a GMail account!
Just remember - a Blogger account can have any valid GMail or non GMail email address. A Blogger / Google account does not have to have a GMail email address.