I agree that both can be used to achieve the same aim. Still there are some difference in their use:
– an extra deck gives me the ability to listen to the sample before playing it (I don’t think this is possible with a sample slot in VDJ)
– an extra deck gives me EQs for the sample
– an extra deck gives me the ability to keychange the sample, which IMO is a big deal since you’d want your sample to mix harmonically with the music already playing
Now the only con I could think of for an extra deck is the fact that most controllers don’t have 4 decks. Of course there are plenty that do, but it might be a bit overkill to buy a 4-deck controller just to use the two extra decks as sample decks.