Having just crammed everything we could of a 10 page paper into 5 pages for a workshop paper, I admit that besides (once again) being annoyed by having to go through this process, I'm not clear for the reasons behind it.
I understand reasons for page limits. Some authors would not self-edit unless forced to do so, and a little regulation arguably makes life better for everyone -- especially the reviewers, but also other readers, and even the authors who wouldn't otherwise take care to edit their text. In some (rare, I think, these days) cases, there might actually be sound logistical publishing reasons for a page limit. Also, one might argue that, as a matter of fairness, authors should be obliged to have a somewhat tight rather than loose upper bound on what they can write.
But 5 double-spaced pages? (Or, for those who use LNCS format, 12 pages, which is roughly the same?) Page 1 is a basic intro, Page 2 gives relevant background, such as prior work, definitions and terminology, Page 3 (for my papers, generally) provides theoretical results (little or no proofs, of course, for any subject with meat -- at best a high-level sketch and discussion), Page 4 and whatever room I can take from Page 5 for experiments (including plots!) and analysis, and what's left of Page 5 for a brief conclusion and the references. Anyone who writes papers can tell there's going to be plenty of stuff you have to cut to make this work. Some of what is cut is just the textured, nuanced commentary that would help the reader; some of it is actually important. (Leading to my irrational anger when I get reviewer comments of the form: "Why didn't you talk about XXX?" when my answer is WE DID, but we had to take it out to meet his arbitrary page limit.)
Papers these days, I guess, are often just advertisements for the work up on the arxiv.* Is this the product of a reasoned argument on the right page limit? I'd like to hear the arguments.**
* I note some conferences have gone the other way, allowing longer papers since it's all electronic and cost of paper is not really a concern. See EC this year...
** Right now the only one I know of is that workshop papers for things like HOT papers should be 5-6 pages, so when you submit the 10-14 page version to the regular conference there will be enough "new" material to justify another publication. I don't find this a good argument, personally.