There are some interesting - and somewhat humbling - counterpoints to Vortex Thing's point of view here. Thank you, guys, you have done a great deal to restore my faith in common sense!
As you say, your training is a very expensive investment, from which you must achieve a decent rate of return. Flying for free abandons that investment, as well as ensuring that the remuneration you receive later in your career will be less than those currently employed. In other words, it's your own patch you're !!!!ting on.
Don't do it!
scroggs