Of course, there's also the incentive to have a peek behind the scenes and see the work in progress, suggestions that are more to the point and much more considered, and to maybe pick up some more acquaintances.
Ah the most common misconception.
Suggestions are more likely to be heard when given by a tester, but they ain't always 'considered' (at least on the side of the suggester
Unfortunately, I'm North American and even worse, a college student, so beta test times would most likely be a fail on my end. Just shooting myself in the foot an all...
What timezone are you in?
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As for me, I had played FH up to 0.7 for roughly 2 years and was asked together with a bulk of other tournament players to test the last FH build and Mappacks for BF42. Most didn't stay for the transition to FH2, I myself stayed since the chance to see a mod develop from zero on a new engine was just great. The entire system was changed and the devs went full pro!
Other than that it's the same motivation as every tester should have, to advance and help the mod (actually it's already a game of its own in my opinion) we all love!
Almost 5 years with the tedabester team now and still the devs manage to amaze me with every new tidbit of info.