The scraper plane offers important advantages over scraping freehand.
During cutting, the angle is maintained by the plane instead of by your hand. Compare it to sharpening a bevel with the aid of a jig vs. freehand.
An even larger advantage is the sole. The sole ensures flatter surfaces, whereas scraping freehand with a card scraper can tend to follow existing peaks and valleys rather than eliminate them. In my experience, this is most noticeable when scraping "curly"-figured wood.
The cabinet scraper (Stanley No.80 and knockoffs) is a type of scraper plane. Since it is more economical than a "real" scraper plane, it's use is more prevalent.
I still use it in certain applications, but it has two disadvantages when compared to the typical scraper plane.
1. Very short sole, so not as well-suited for maintaining flatness.
2. Still requires two-handed operation, whereas a block-style plane can be used with one hand.
Another option is to take an existing plane and convert it.
I haven't seen any inserts to convert a block plane, but if you have a No. 4 (or larger) you can get the benefits that the larger scraper planes offer over the cabinet scraper without the $150+ price tag.
I have two planes that have been outfitted with the insert offered by Lee Valley, and they work great.
http://www.leevalley.com/US/Wood/page.a ... 10&p=32635
And as always, for those confident in their understanding of plane design, a scraper plane is as easily made as any other shop-made plane.