If Clinton's IT aide is pleading the Fifth to avoid incriminating himself, that means he has something to hide, something that will incriminate him and Hillary Clinton. If he truly did nothing wrong, he would gladly disclose all of the information that is needed. This may not not incriminate Clinton in the eyes of the law, but it just shows that she did do something wrong.
Yes, it's damaging to her case, but it's not incriminating unless we know what it is that he's refusing to admit to. Sounds like a catch 22 to me. If he speaks, he incriminates himself and they bring charges against him (which is the purpose for pleading the fifth), AND his testimony could incriminate Clinton. I would be very suspicious, and think that the prosecution is on the right track, but I'm thinking that without his testimony, they have no legal leg to stand on.
The law allows witnesses to plead the Fifth so that they are not forced to incriminate themselves. If Clinton's IT aid refuses to speak, he is protecting himself - not Clinton. Sadly though, as the email server controversy goes on, many will see his refusal to talk as proof that Clinton is guilty.
While it is very easy to draw the conclusion that Clinton IT aide pleading the Fifth as the email server controversy continues does incriminates Clinton as guilty, the reality is that it does not. The fifth is a constitutional right and should not be held against anyone when it is used. That said, Clinton is as guilty as the day is long. You don't even need the IT's testimony.