The biggest downfall I see with reiser (or other linux file systems), is that if I need more space, I can't do it. If I could use NSS for GroupWise instead, it's easy to expand the pool on the fly, immediately giving more space to GroupWise. I realize that most hardcore linux people are not familiar with NSS or Novell's traditional services, so they don't understand many of the benefits that Novell people have taken for granted for numerous years.
Even if GroupWise on NSS has a 5% degradation compared to Reiser (Assuming OES2), the extra flexibility for future expansion easily outweighs the performance. 5% is nothing in my book.