They need help with creating a game but they don't have any money to pay potential team-members up front, so they look for people who are willing to volunteer their time in exchange for a share of any money that gets made if the game is released. When making this sort of offer there seem to be two common approaches to how the payments will be structured:
- The team leader has some idea of what they think a particular job is worth, and they offer a fixed share of any money made based on that. As an example, they might think creating the soundtrack is 10% of the work, and therefore offer to give that amount to a composer. It isn't uncommon in this case for the percentages to be somewhat poorly thought out; the amount offered often indicates poor or no idea of how much work a particular job entails (usually leading to offers of tiny portions that wouldn't be worth the effort even for a guaranteed success), and sadly it isn't uncommon to see the total number of shares being offered for the various roles totally more than 100% of money made. It's also often not very clear what would happen if someone contributes but leaves before the project is completed.
- Realising the above can be unfair, a system is proposed where contributors will be paid a share of any money made based on the amount of work put in. In this case it's almost always unclear exactly how this will be calculated; what if one artist contributes only a single really complex model, whilst another contributes many models that are very simple -- how should shares of money be split? How much is one piece of music worth? Again, it's also usually not clear how the situation of someone contributing but not seeing the project through to completion would be handled.
The second option above is essentially what has been proposed to you, although with the addition that the amount of money given will vary based on experience; which as with most of the above, sounds reasonable on the surface but has a number of problems. What if a seasoned industry veteran joins the team and contributes a single simple model to the project, whilst a wet-behind-the-ears beginner puts hours on end into producing many many models -- as it is described, the system in place would pay the industry veteran more money even though they don't really deserve it.
The point I've been making with all these example situations is that such a share of profit is almost always very poorly defined; this is a bad thing for everyone involved, and is something you should endeavour to have corrected before taking part. Preferably, how payments will be split amongst team members, under what conditions they can be given or taken away, etc. should all be outlined in a proper legal document drawn up by a lawyer.
Failing that, everything should be decided up front before any work is done, and everything should be written up clearly and concisely in a plain-English agreement so that there are no misunderstandings about how things will work. Note that I'm not endorsing this approach -- proper legal agreements should absolutely be used -- but if that isn't going to happen, don't try to write your own legal documents, as the language used has very specific meanings and you will likely make costly mistakes; just use plain English and be sure that all parties understand the conditions before proceeding.
As for whether such a system is a good idea or not?
It's all about risk. Normally the studio/business/team takes on the risk by paying out money up-front to have work done; if the game is a success they will (hopefully) make that money back, but if the project isn't completed or doesn't sell very well that money will be lost. In the case of a profit-share or royalties the risk is shifted to the team members; you're working for free (or sometimes for a reduced up-front amount), and if the project is a success you'll hopefully earn enough money to compensate for the time and effort you've put in.
In this particular case it sounds like you want the experience of working with Hero Engine, which helps to balance out the fact that you're pretty unlikely to actually earn any money. Is it worthwhile? Depends on your own situation. Personally I'd push for at least some payment up-front as well.
Hope that helps!