Depends on your definition of \"mastering\". There's no reason you can't use plugins exclusively to master with. But the term is very loosely interpreted these days. Import into Wavelab, squash the hell of out it with Compressor X and Limiter Y, add some EQ somewhere in the chain for the hell of it, sequence the tracks, add fades, add markers, burn to CD, send to DiscMakers. To some, this is mastering.
I \"mastered\" an album a couple of weeks ago using just plugins. However, I have a solid grasp of audio principals and know when and why to use the tools at my disposal. Is it going to sound like it was done at a legitimate commercial mastering facility? Nope. But for my client who only had $250 to spend to master a whole album, I gave him $250 worth of my time, skills and experience to produce something that sounds acceptable and is \"in the ballpark\". Considering the budget and resources he had for recording and mixing, my \"mastering\" was right in line with the rest of his project.