Actually, I have seen this. There are at least two very well know groups (that I will refrain from naming) that will not accept you into the group, or remove you from the group, if they find out you worked as an expert witness for the defense in a criminal matter. If you talk to the group members the prevailing opinion is that when you work for the defense, almost 100% of the time you are helping them make their case by challenging the methods or ability of another forensics analyst. If you are attacking their methods (tools, the science behind data forensics, standard approaches, etc), then you are actually attacking the entire practice of forensics which is bad for the community. If you attack the ability of the other analyst, then this is often viewed as a personal attack against someone that was trying to catch a crook. I don't necessarily agree with these arguments, but I hear them a lot. With that being said, if someone screwed up the case, then they screwed up the case. Period. Also, if you do your own analysis and can present evidence that is valid and relevant to the case (ex. you find out that someone's system was actually hacked into and the illegal activities might not have been performed by the system owner but by the intruder) then that should absolutely be presented in court. However, whatever your motivation might be, as soon as you sit on the other side of the isle there are just going to be repercussions.
CISSP, CISM, CISA, GCIH, GREM, CEH, HMFIC, KTHXBIROFLCOPTER