Re: Liquidity... The fact that a service is immediately useful to somebody (and has a willing customer) does not prove that it's a net productive behavior for the society, as a whole. That's just how the field of economics defines it, and the practioners widely agree that "providing liquidity" falls into that category.
Now, it sounds like you may be trying to defend the morality of rent-seeking behavior... If that's the case, I wish you good luck in your argument, with somebody besides me. I have no dog in that fight.
The real issue is when owners seek benefits of ownership that are not correlated with efficiency or productivity. For example grants and subsidies, inflation of rents though monopolistic practices or opportunism. In fact all monopolistic profit inflation is renty in a way, regardless of what the business model is, because it's extracting extra profits from simply exercising control of something in excess of the economic value provided. It's exercising the power of incumbency that's the problem.
Some activities classed as renty can be economically beneficial. It’s not all pure usury, but activities that seek to increase rent revenue without increasing the value provided are a problem and that’s the ‘rent seeking’ part. I’ve no problem with fair value rents, but rents should be as low as the market can reasonably bear as excess rents are essentially a tax on production.