One member of our writing group has been keeping journals for many years, and has recently been going through them and creating summaries, to make it easier, down the road, for her family to access and read her journals. Wow! That's dedication. While I firmly believe her family will want to read the full text of her journals, witness her world, and see the world through her eyes, the summary would highlight the main events noted, allowing readers to see what information is included within a single journal. She is also creating summaries of other experiences she has written about and materials she has inherited from her own ancestors, again to create a more ready access.
Summarizing is especially good when you have a lot of disparate material to present. Journals are, for the most part, written chronologically (even is a time lapse between entries) and they have context. Military records and other types of documents might not be as easy to read, so it makes sense to extract pertinent information and incorporate that into a life story or a chronology of someone's life. This is what professional genealogists do in their reports. They summarize the main features of the material researched, carefully documenting where the information derived, and create a nice readable document. But the originals are still there for reference. Summarizing journals are something else.
While I have not created summaries my own journals, I have created a table of contents, that does something of the same thing, indicating the dates covered within a particular journal and the main events covered during that time period -- a wedding, the birth of a child, graduation, etc. The only problem, I think, in condensing, is losing the thoughts and feelings that tend to emerge as we write, which is, after all, the essence journal writing. In summarizing, we might tend to highlight the upbeat and positive, glossing over life's challenges and how we managed.
But so long as the summary does not "replace" the full text -- in other words, we keep the original, we can be fairly certain those who follow will want to read it all and discover, line upon line -- and between the lines -- the experiences and lessons of a life lived.