It is essentially a question of backwards compatibility.  The author (and the author's publisher) must weigh the importance of easing the reader back into the story after a potentially long absence (even writers of lighter fantasy series only release a book or two a year, and authors of epic series routinely take several years to finish each book) against the desire for a clean, progressive narrative. 

The structure of Hobb's trilogies is fairly representative of the majority of serial fantasy: the story is essentially one long novel broken into several volumes for purposes of authorial efficiency and publishing.  But the fact that Book Three of your favorite fantasy series takes up right where Book Two left off means that the publishing gap is that much more obvious.  You, the reader, have waited a year to know what happens two days after the end of Book Two.  Given this pragmatic necessity, authors and especially publishers must necessarily feel obligated to do some "catching up."  Unfortunately, it is difficult to do this stylishly.  Think of your favorite TV show: when it airs once a week during a current season, it is useful to watch the introductory recaps ("Last week on CSI....").  But when you watch the episodes back to back on DVD, the constant summarization becomes an annoyance.  Likewise, fantasy recaps are really only useful to those readers who have waited a significant amount of time between books.  A little summarization is great when you are reading Book Seven of The Enormous Epic Fantasy Series after a three year wait for its publication, but if you just finished Book Six yesterday you are probably up to speed. 

So the conscientious fantasy author essentially has three choices: (1) pander to those readers who have been waiting for the newest book for a long time and recap the action so far, (2) pander to those readers enjoying the work later on with no foreseeable gaps in reading and maintain the integrity of the work as a whole, or (3) attempt some sort of compromise of the two, weaving the recap into the story deftly enough to inform the forgetful and avoid boring those who remember.  Unfortunately, the third option seems to be the one most often chosen.

As one who has little to no patience for the (often awkward) reintroduction of the plot, I prefer an approach that is both more overt and less intrusive: a separate introduction. 

Tad Williams, in his Memory, Sorrow, and Thorn trilogy, simply added a brief, external plot synopsis to each book in the series.  This approach seems to work for all parties.  Those who want a recap have one and those who do not can easily skip to the first page of the actual story without having to miss anything, as the synopses are introductory only and do not contain any new information.

Some might argue that this approach lacks style and somehow cheapens the art, but concern over a reader's ability to keep up with an ongoing story after a publication gap is essentially a marketing problem anyway.  Better to put things out in the open, so that the work itself doesn't suffer.  Either go all out and admit that you're worried the reader might get lost in the story, or scrap the recap altogether and leave it to the reader to figure out.  Fantasy readers expect a lot from their writers; writers should expect a lot from their readers. 

It is not necessarily the writer's job to make his work easy for the reader to swallow.  While it is a modern presumption that consumers have only a thirty second attention span, writers should not think of their readers as consumers.  But if they do, they should presume them to be self-reliant; they have, after all, chosen to read a book rather than watch TV.