I too believe they have a part in a divrsified portfolio and...

I too believe they have a part in a divrsified portfolio and agree with gopher (TU).

No, they aren't cash. Yes, they are pretty liquid.

Just check out what happened around GFC to some of these, they can tend to mirror the stock in volatile times (check ELDPA).

Also, there can be scope for capital gains if bought low and sold high, especially ones that have an end date.
Anything that pays a higher income pays a higher income for a reason. Just understand those reasons.

Hope that helps