While it might be convenient to have lots of placeholders, the result is that pages would be constructed in a way that requires more and more hits to the database to produce the page content. It is much better for performance for a page to have 1 instance of html content feature and do layout inside that content instance (which can be facilitated by the use of content templates where you could have some editable layouts already created for your end users) instead of having 5 different intances of html content feature on the page which would result in 2 hits to the db per instance, one for the content and one for the instance settings. If you need more than one kind of feature on a page then of course you need separate instances of each feature. Having more instances of content placeholders would encourage users to do things that it is better not to do.
Best,
Joe