A lot of players never uses leves and sit on max allowance pretty much all the time.
Yes, it takes 10 leves to actually get around the same EXP as 5 regular leves... but you save yourself quite a lot of time.

These leves are meant to be used that way. If you always use all your leves on regular ones, then Big Temple leves are not for you.
However, if you never did your leves anyway, doing a big leve from time to time completly worth it.

Keep in mind that with at least 4 roulettes to do per day to get your tomestone of Law, plus your daily hunts, plus your 12 daily beast quests, it takes a lot of time and not every player can then do all their regular leves.

These big temple leves are designed to make you do around 5 leves in one go (saving a bunch of time), while costing double allowance as a counterpart.