Most movies are smaller than 9 GB and therefore fit on a single disc (double-layer).
Only a few films go over this limit (e.g. LOTR), and need more than one disc. Marketing departments are also keen to release 'special edition' high-priced multi-disc sets, where 1 disc is used for the film, and the other(s) for the dubious extras.
A few years ago, some studios used only double-sided discs, rather than the more expensive double-layer discs. I was rather dissapointed when one of the first DVDs I saw was 'Armageddon' and you had to flip the disc over half way through. [WB eventually saw sense and, in fact, offered to exchange the double-sided discs for double layer ones].
The small label area (making identification less obvious), and increased fragility of double sided discs, have certainly put me off them. At the same time, they offer little advantage over 2 seperate discs - you still can't watch from end-to-end without intervention.