This question is wholly determined by your answer to the question "Is it ethical to download copyright music/software without paying?"; the specific circumstances surrounding a particular example merely disguise the general question that's being asked.
Choice of specifics
If you think it's ethical to download without paying, you can find a set of circumstances involving no knowledge and negligible harm for the company and both dire need and lack of resources for the downloader. Conversely if you think it's unethical, you can find a set of circumstances involving harm for the author and little benefit to the downloader.
Similarly you can come up with analogies that lean one way or the other - for: libraries promoting reading fostering more book purchases, against: joyriding (when returned to owner's house with a full tank of petrol and keys posted through door).
Analogies that seem parallel to me: using a cracked hence free subscription card to access satellite television for a week to decide whether you want to pay for a subscription.
I think looking to the consequences of a single event is perhaps misleading when determining the ethics, particularly since ethics is intended as a guide to future behaviour rather than a justification of a single event. Thus again, the question "is it ethically OK this time" should really just be "is it OK ethically".
Does the trial basis change the ethics?
I'm not convinced that calling it a demo changes whether it's ethical or unethical - a temporary injustice is unjust, but temporarily, and a temporary justice is just, but temporarily. They differ in degree but not fundamentals here, since there is no physical item to keep or return.
You might want to argue that in the event that the software/music is purchased, the copyright holder has obtained payment, so that this is fine, but unless you can conclude that it's ethical when payment is never made, the answer becomes "only if you pay", which sounds like a "no" to generally downloading without paying! Thus as I said, this really boils down to whether it's ethical to download without paying full stop.
Can you weigh potential benefits against harm?
Of course, if it's ethical generally, it's ethical to do on a trial basis, but if it's unethical generally, it's unethical to do temporarily, since it's not it's not generally true that a potential future benefit outweighs a harm.
As a deliberately extreme example, consider a man having a one-night stand without his partner ever finding out. He might argue that she benefited from all the extra attention he lavished on her because of his secret guilt and from some improved sexual technique he developed from this event, so that she has no idea it happened and has benefited. This argument doesn't really address the question of whether his behaviour was ethical.
To be clear, I'm not saying that downloading copyright material is morally equivalent or even comparable to having an affair, I'm showing how the reasoning "they may well benefit, and will never know, so it's ethical" is flawed.
(Notice that if the couple agree they're free to sleep with other people at any point, the example becomes rather different.)
Thus we must come again to the conclusion that the ethics of trial downloading match the ethics of downloading generally.
Two philosophical perspectives you might want to try out on the problem
Kant would argue that if everyone failed to pay for music or software, harm would result (job losses, less money spent in making popular music), so since the behaviour is not generalisable, it cannot be a moral rule to allow it.
At the opposite (anarchist) end of the argument, Proudhon would argue that property is inherently theft, that the copyright holder had no right to charge for the download, that this wrong was exacerbated by its costlessness, so it is ethical to peacefully circumvent this unethical charge.