It was only kept for Legacy files, that is version 10 and even older files.
That is not true. I created a sample text in AI CS3 containing a large paragraph. I then open that CS3 made file in CS5.1. The layout is kept exactly the same. If I then double-click on that text to start editing, I get a message "text was created in a previous version of Illustrator and its going to change the layout: Do you want to continue?". If I click Yes, then the text is being reflown according to new AI CS5.1 text engine (which of course make the layout change).
When I open the same CS3-made file in AI CC, I do not get any message anymore, and the text layout is just changed immediately. Does that mean that text layout will now change for every version ?
Suppose I want to make a text box, and I do some trapping on that text on some areas. Now, when I upgrade the AI , all my traps are not correct anymore, and I will need to redo them every time I upgrade. Note: this was NEVER the case in any AI CSxx version: I could always upgrade and I could always reuse the files without the need to look at all live text. Of course, if I wanted to change something in the text, then its logical that the reflow gets change completely and that is acceptable. But if I want to open an old file, and just want to make a small correction, I cannot do that anymore without a big work of looking at all text blocks.