They have been in, but sounded less limited now, that said I do totally understand why they do limit them, if someone who raided every night for a month or two ended up with all the best gear pretty quick you'd lose that playerbase because they would have no reason to still play.
Colluseum is an interesting idea, much easier and more accessable and you can easily add to it just by copying the area to another door that opens, hardcore raiders would be happy there for a fair while if the loot was good enough