A legal reader offers an explanation and a suggestion for the Dems:
1) The 5th amendment gives protection against having to testify in any situation where truthful answers might incriminate.
2) Monica Goodling cannot possibly have substantive guilt here; she is 33 and was working in a subordinate role for aggressive bosses. If she needs protection, it can only be from a technical guilt whose moral weight rightly falls on her superiors. Most likely, she and her lawyers fear that any perceived problems with her prep session for McNulty's testimony could be construed as indirect "misleading" of Congress, and - by a stretch no one would be likely to make - charged as such.
3) The solution is therefore to offer her immunity from prosecution for any actions her testimony might disclose, and then compel that testimony through subpoena. This is fair, because even if she could be found participating in a conspiracy to mislead Congress, what we already know of the situation makes it extremely unlikely that she was the originator of any such conspiracy. It is also effective, because while it would immunize her from jeopardy for anything she has done already, it would NOT immunize her from perjury charges should she fail, now, to tell the truth. And she is likely to have knowledge of some pretty central truths.