Working in the web app world, I frequently get asked, often by managers who aren't thinking about the product and SOX compliance, if it is OK to plop just a jsp file or a jar instead of rebuilding the entire war each time.
My answer is always "No!". There are a number of reasons why, but most simply is that doing so defeats the basic tenets of release engineering: reproducability, traceability, accountability.
As a simple example that recently occurred in my own job, we were starting to upgrade a pair of server that had not been touched in nearly three years. We started in on the first of the pair however we soon found that the code needed to be reverted. When we redeployed the backup of the war file, the code was exhibiting incorrect behavior, especially compared to its twin. Turns out that soon after the deployment three years earlier, a developer also deployed a single JSP file into the exploded war directory. Three years go by, developer has moved on, managers have forgotten everything, two different release engineers moved on. The developer was playing around with a production system, didn't document what happened and didn't re-release the product to correct the situation. As a result, the product could have been in jeopardy if we had not been more tentative and worked on only one server at a time (another topic of discussion: a lot of developers want me to deploy to all the servers at once). By only working on one, we could compare the old war with the changed war on its twin.
This anecdote illustrates one major issue with partial deployments. There was no accountability for the product: none of the manager, developer or the release engineer at the time of the first release held themselves accountable. There was not tracability: no documentation was kept, no build records, no labeling - just a file plopped down into a directory. No reproducability: the environment could not be reproduced without looking at the twin server. If that twin production server hadn't existed, then there would have been nothing to revert to.
Full deployments from a single, release distribution build lead to all of these tenets. Anything less breaks release engineering practices.