Is there proof that slow wave sleep is beneficial?
There have been some studies that attempted to interfere with slow wave sleep while keeping other sleep parameters intact, e.g. sleep duration, REM-sleep, stage 1-2 sleep duration, nocturnal awakenings… Such studies have revealed the importance of slow wave sleep (or at least a complete undisrupted sleep cycle consisting of the different sleep stages including REM and slow wave sleep) for certain memory functions. In addition memory reactivation (or memory replay) as seen during slow wave sleep provides additional evidence for the importance of slow wave sleep in memory consolidation.
An example of a study manipulating one parameter of sleep (i.e. amount of slow wave sleep) while keeping other sleep parameters intact, is an acoustic perturbation experiment conducted in humans . When slow waves were detected an acoustic stimulus was presented, gradually increasing in volume until stage 1-2 sleep was reached and acoustic perturbation was terminated, thus preventing the subject from awakening. Although sleep duration was maintained, as well as the amount of REM-sleep and the number of sleep state transitions, performance on a visual memory encoding task was negatively affected (this task consisted of responding with either ‘yes’ or ‘no’ if a presented image was previously seen in an initial set of 50 images).
Neuroimaging data revealed that during encoding of items that were later recalled successfully, the right anterior hippocampus had lower encoding-related activation following shallow sleep as opposed to normal sleep (see figure). Thus slow-wave sleep interferes with hippocampal functioning and can affect tasks that rely heavily on this brain structure (such as the visual memory encoding task).
Performance on an implicit procedural learning task (a 4-choice serial reaction time task ), a task that doesn’t rely on hippocampal function, wasn’t affected.
Memory reactivation, or so called memory replay, is another phenomenon seen during slow wave sleep, that provides evidence of the importance of this sleep stage in memory consolidation. The figure below (Wilson et al., 1994 ) reveals how place cells (as presented by the individual dots in the circle; http://en.wikipedia.org/wiki/Place_cell) that fired together (synchronised activity) during a spatial behavioral task in rats (“run”-phase), also had an increased tendency to fire together during subsequent slow wave sleep (“post”-phase), while this was not the case during slow wave sleep preceding the spatial task (“pre”-phase).
Although I have given some examples of the importance of slow wave sleep, some conceptual frameworks  emphasize the complementary or sequential contributions of slow wave sleep and REM-sleep to memory consolidation, and thus the individual sleep cycles and NREM-REM sleep transitions as such to be of importance. So although slow wave sleep has been shown to have a vital function in memory consolidation, this should be seen in the context of the sleep architecture as a whole, as well as individual sleep characteristics/parameters (e.g. hippocampal sharp waves/ripples) that could convey information about how memory consolidation is acquired.
 van der Werf, Y.D. et al. Sleep benefits subsequent hippocampal functioning. Nat Neurosci 12, 122–123 (2009).
 Robertson, E.M. The serial reaction time task: implicit motor skill learning? The Journal of Neuroscience 27 (38), 10073-10075 (2007) (opens pdf)
-> see section about serial reaction time task for information about this procedure
 Wilson, M.A. & McNaughton, B.L. Reactivation of hippocampal ensemble memories during sleep. Science 265, 676–679 (1994). (opens pdf)
 Diekelmann, S. & Born, J. The memory function of sleep. Nature Reviews Neuroscience 11, 114–126 (2010).
 Born, J. Slow-wave sleep and the consolidation of long-term memory. World J Biol Psychiatry 11, 16–21 (2010).
-> provides conceptual framework and reviews evidence on the specific role of slow wave sleep in memory consolidation