Redeye I have to agree with KMS. working for no pay does no one in the industry any good at all. Believe me when I was starting out I was very very tempted to try this route but held out for paid work and got it in the end. Now I have the experience and the pay to go with it.
In the fixed wing arena they have made the mistake of working for free and even paying for their own heavy metal type ratings. The trouble now is that it is fast becoming standard in order to get a job for you to pay for the rating or the job will go to someone else who has.
Imagine you have just spent the last couple of years studying like mad for your exams, paid for your PPL and hours building plus cpl flight course. You go for a job and don't get it because someone else is doing it for free. or maybe you are ten years into your flying and you find that your work and pay have been cut due to someone flying the easier tasks for free. not a lot of fun for you.
Plus don't think that once you have experience you will go from being a free member of staff to a paid one. That type of company will just replace you with another new pilot who will work for free for a while. I have nothing against getting free flying but not if it is at the expense of a fellow pilots wage packet.... one day hopefully that fellow pilot will be you. There are ways to get free flying that are not at others expense if you are lucky and willing to work for it.
Sorry MPD rant over...
as to the original point of the thread then yes there is an awful lot that you can learn from trial lessons especially if you let them have the controls for as much of the flight as you can......and remember never trust a student they are all trying to kill you. It is normally your "good" student who you relax with and then catches you out. Trial lessons are a great way to learn how to judge people and how they behave and it is always fun to see how much they enjoy it.
Good luck with the flying