The themed PPV events are a result of lazy booking. However, it does help generate some interest in the shows. For example if it's October and you can only afford to buy two PPV events for the remainder of the year, you know that Hell In a Cell is next followed by Survivor Series and TLC. I'd then save money in October and purchase the following two shows in Survivor Series and TLC. A lot of people disagree with the concept, but WWE might be looking at even lower buyrates without the gimmick themed shows of today.
Back when Judgment Day, No Mercy, etc, were all still around.... I rarely ever purchased them. WWE made it clear that these shows were less important than the Big 4 or even Night of Champions. So why should we as fans care either? Sure, Hell In a Cell tends to be one of the worst events each year, but at least you know what you are getting and therefore it helps you decide if it would be worth buying or not. If I heard "at No Mercy it will be a World Heavyweight Championship match in Hell In a Cell!" it would not make me buy it. Knowing ahead of time all year that this match is coming at a show with that theme, it helps me make that decision rather than an immediate "no" like the generic older shows used to get from me.
Hell In a Cell I can see them getting rid of completely. TLC on the other hand should stick around. It's 4 gimmicks in one, which is why I look forward to it each year. Would it be nice to see more Ladder matches? Sure, but I know I am in for at least 4 gimmick matches all under different gimmicks at that event. Tables, Ladder, Chairs, and a TLC match. There's no reason to ever end that show. It also wraps up feuds for the year. Hell In a Cell, on the other hand, is just an annual waste of money like the generic older shows were. It's a small step above them due to letting people know what to expect months in advance.