For those that keep badgering on about how they expect this, that and the other from Monarch, all it makes me think is... Stuck-up snob!
If the airlines didn't hire in these extra aircraft, they could potentially lose business by not being able to provide enough flights, ticket prices may be higher if they had to by aircraft rather than leasing (Defeating the point of low cost airline) and ultimately, if the airline is not making a profit, then it'll go bust, so it'd be game over anyway!!
The level of service is the same and the end result is the same - you get to your destination. The only thing that is different is the fact you are on a 737 instead of an A320.
If you don't want to run the risk of being flown on a lease aircraft when booking with a low-cost airline...fly with a company such as BA.