Back on topic...
The concept of the post is to take a essentially unused arena, and provide a gateway (by making if free) towards paying subscribers.
Pros:
More players in WW1, many current subscribers would love to fly here, but there is a lack of someone to fight. This might solve that.
As Bruv stated, it allows interaction from a prospective AHaddict, with individuals who are passionate about the game. This could convince them to join. (as previously discussed in the post)
More people joining (because, hey! its free) equates to more discussion, which generates more interest, which could make AH more relevant, and prominent in the gaming world. (in terms of relevence in advertising) (think google search)
Any advertising is a numbers game. Period. People like free. Period. This is a relatively low cost solution for advertisement. As a numbers game some will choose not too join, but some will. It would be impossible to not see lift in subscribers from sheer statics. (whether this is financially viable, or interferes with other marketing is a separate issue)
*Rant warning*
...As a personal Pro:, I am so (excuse the grammer

crap excited about flight, that I would go out of my way by far to promote AH as it serves as a fantastic outlet for my passion of flying. I can't afford to pilot a fighter plane, but I'll be dammed if I can't get as close to it as I can. (I flew in a cessna 152 once, for a half hour, and I was instantly hooked) How would I not want to let others experience a entirely different level of gaming? (niche audience, rather than mass marketed i.e. MW3, or whatever that World of Airplan...who cares...
Cons:
There is no way that a free subscriber to WW1 online arena will get a realistic representation of what actual gameplay is like.
As bruv stated, and I believe in, many players will welcome new players and want to get them excited about the game! But alot will not as well. It's an online game...(squeekers, noobs, newbs, nubs, Ho'ers, vulchers, pickers, alt monkeys, runners, spit dweebs, runstangs, lgays. etc.) I would be concerned in what environment a free player is getting into, which...
...as a result may turn off alot of people since this is not by any means a game that one can get "good" at easily. (that's one of it's charms) but in terms of recruitment it is likely not.
It might just cost something to host more players for free?
It may dissuade current subscribers and cause the issue of..."But I PAY FOR THIS, WHY DON'T THEY"! I would like to think that wont be an issue, but I believe it will. (Remember every country hordes, and gangs up, but your's of course does not)
If HTC allows one online aspect for free...then other stuff should be free too! ^(like the above post)
I still believe this to be a "niche game" but there are thousands of people that want to play, but haven't heard of us. After rethinking, I'm not certain WW1 should be free, but that the solution to more players lies on a much more local level.