From what I've read, taking the Fifth may not (in many circumstances) preclude taking a negative inference from doing so, however.  It may be very safe to do so in this case.

I think it's useful if you're being questioned about something where you're unclear whether it's an offence or not.

You've been subpoenaed to testify in someone else's trial. You're asked a a suspiciously arcane question about the specifics of what you did on a certain day. (As a dumb example: maybe you were sightseeing in Florida and you drove past a fenced landing strip with a really cool plane you'd never seen before. You took a photo with your phone. Now you're being asked why you were there and did you take any photos.) You didn't think you did anything wrong but now you're worried you might have done. You really don't know. 

I think it makes sense that the law cannot compel you to give an answer that might possibly incriminate you.

One of my friends wondered if Trump was the one who blew the whistle on the FBI raid, positing that Trump was the person who announced it, no media were alerted, as in previous related cases.  That seems farfetched but I did note Trump's fundraising hit record levels after the raid, and the entire media seems to be talking about him again.  

No argument about Trump: I wasn't attempting to provide an explanation or excuse for him, simply a follow on comment about why the fifth exists in general and how it might not imply guilt.

As for ignorance being no excuse for non-observance, once again no argument other than no one can be expected to have a perfect knowledge of all laws and my example (admittedly dumb as it is) was supposed to illustrate a case of ambiguity. You may not have committed a crime. Maybe you have broken some statute. But it's not very nice if you are compelled into incriminating yourself if you have inadvertently done so.

Meanwhile, Trump is whining about made up crap, complaining that they didn't go after Obama for 33 Million pages (seriously!) of documents he supposedly took when he left office.

That may very well turn out to be the case, but we don't know that yet, and we won't know unless/until the warrant and supporting affidavit are revealed.  Personally, I don't trust the media (Fox, CNN, or any of them) with respect to information about the documents that has been leaked. 

Garland has asked the judge to unseal the warrant, along with 2 related documents (hopefully one of them will be the affidavit). So I do believe that eventually we'll see it.  As to whether the search was appropriate, I'll withhold judgment until then.  

If it turns out there was good cause to search Mar-a-Lago (which I hope was the case, because I want to believe this was not politically motivated) then I don't have a problem with it.  I don't think a former president should be treated more favorably than any other citizen.  

Some things I'll be looking for:

- Specificity.  Do they know what documents they are looking for?

- Classified information

- Evidence that Trump was not cooperating with the subpoena, or that there was reason to believe documents would be destroyed, rather than turned over.