There is a property called the "Co-Effecience of Expansion and Contraction".
What this means is that while the plaster will expand and contract at one rate the drywall will expand and contract at another rate. Resulting in cracking and failure. The same thing goes for when you apply one type of material (say Wood) up against another type (say brick). The two varying densities in materials will cause a failure between the two.
Also, this principle applies to different material densities being applied over the top of each other. So, This being the case. You wouldn't want to apply drwall mud or spackling over the top of plaster (different densities) or next to each other.
There are a few ways to repair the area and ensure that it will last the longest time possible.
One is to use "Bule Board" (USG). And use a three coat system.
Another method would be to use lath and plaster.
Both methods will most likely take a professional plasterer to do. However if you want to go the DIY way let me know. I'll help as much as I can.