The dimming effect on windows in the Present Windows effect makes it harder to immediately recognize their contents, as they have less contrast with the dimmed background. This is compounded by the location of title bars (if they're enabled), right in the middle of the windows, obscuring their contents. The title bar location also makes the title bars harder to read too.

Here are some screenshots of similar effects in GNOME, Windows 10, and MacOS:
https://upload.wikimedia.org/wikipedia/en/c/c2/Shows_Overview_mode_%28%22Activities%22%29_in_GNOME_3.8.png
https://images.techhive.com/images/article/2015/07/taskview-100598415-orig.png
https://cdn.arstechnica.net/2011/07/04/lion/mission-control-big.png

As you can see, the dimming (if present) is not applied to the windows, and the title bars are either to the bottom or top. This is better for readability. I think that at very least, an option to apply this setting in the effect would improve KDE's looks and usability.