Blocked paragraphs are similar to all caps. They are are also harder to read; type one to see for yourself.
An indented paragraph uses white space to break up the columns of text and signals to the reader that a new paragraph begins here.
When you block paragraphs, you have to put a one-line space between paragraphs to give the eye the same relief as an indentation (in 8-point type, you get nine lines to the column inch). And the reader still doesn't have the same sense of new paragraph beginning.
Like most of these typographical rules, it comes back to habit. When books, newspapers and magazines start using blocked paragraphs, so will I.
Until then, I'll do what people are used to and familiar with. The typography in an ad should be invisible to the reader. You don't want anything to distract the reader's attention from your message.