If they can figure out how to keep people past that first 20 minutes, this idea makes some sense.
That is a legitimate point, but could be partially related. If I thought this is a game I could continue to play for a long time, I might be more willing to figure it out. If I know it is a limited trial, and I believe at that time I probably wouldn't want to pay, I might just not be motivated. But they need to improve the 20 minute thing in any case. But if it took 6 weeks to turn a skeptical player into a log-term subscriber instead of two weeks, wouldn't it be worth it? How much does it actually cost HTC to float a free player vs advertising costs?
As to Shuffler's point, a two week trial does not increase target variety. Two-weekers are not required to fly unpopular planes. Two-weekers are not funneled into off-time slots. Free players could be regulated by providing different numbers of opening at different times, encouraging them to play at times you need back fill.
The two near-model competitors (Warbirds, WWIIOnline) to AHIII both have free level access to the main play area with limited planeset/weapons.
More importantly, I'd say about 90% of the software in the mobile app stores have adapted the model of a feature limited free version you can use forever, with a paid version upgrade. There are some that a pay up front and some that have one time limited trials, but the VAST majority have a free version you can use forever, but a paid version that has more stuff (or no ads). That has proven to be the most productive model. And more importantly, I'd guess the mobile app stores are where the current generation if forming it's expectations of software pay models.