I understand how ending up with bitcoin from an address related to a an address linked back to illegal or immoral activity could make you seem suspicious of something and how the receipt can claim the money is sourced from a mixer. However, couldn't this also be used the other way around to say that the money was gotten from a mixer and therefore it was originally from an illegal activity/money laundering.
That is actually what the Dutch government wants to do: if you have Bitcoins from a mixer, they want that to be considered suspicious by default. I don't think that law has passed yet though.
If they reverse the normal procedure where they have to prove you did something wrong into you having to prove you're innocent, then yes, mixing doesn't help you.
Of course, this is a backwards way of thinking and completely wrong, but just an example of times in which forces such as the government are looking for reasons to steal the money and this might not fully help. It is always good to have the option, however.
I wondered about something else (when BitMixer closed): suppose you end up in court for whatever reason, and you show a signature from either BitMixer or ChipMixer. That signature is supposed to prove you got money from the mixer, so you're not involved in the crime they are trying to convinct you for.
BixMixer used that signature as a selling point, but I wonder if there has ever been a lawsuit in which it was actually used. And if so, it comes down to a judge trusting an anonymous source (BitMixer or ChipMixer). Playing devil's advocate you could argue BitMixer can still use their address to sign from after closing, and that signature can say anything they want.