Promotions work, which is why various companies engage in them. It brings people in the door. Even if you can't keep all of them, there are a few who will stay. Actually maybe more than a few. I'm a writer who has also utilized this kind of advertising promotion, so I can definitely attest to the fact that it really works. Everyone loves a freebie. I have a series of romance novels. When I give away the first one for free there is an influx of activity resulting in repeat customers who come back to buy part 2 and 3.
Yes, I think that these types of promotions result in a permanent increase in customers because customers can try Taco Bell's products for the first time and fall in love with the food. Everybody loves free food, and this promotion helps bring different kinds of people to Taco Bell that don't usually go.
These types of promotions only work while said promotion is going on. A lot of times people just want some free food. As for long term customers the regulars stay the same and those people that came for the free might come back but a lot of times they are just there for the free food.
Promotions only help so much to increase customers because most people are already familiar with Taco Bell. They already know whether they want to eat there or not. Overall it's not an effective strategy unless it is a new restaurant few people have heard of. Of course the promotion might help retain current customers.