as said the big problem with casual raiding is while you can have it, due to the way people play being erratic and changing from one week to the next they will see some content, if people are changing each week you need to gear up a bigger pool of people, meaning one week you could have a really strong time with players who turn up quite often then the next week you get a group that is mainly made from the less active members so the entire group takes a step back and can't do as much.
no one is saying casual raiding shouldn't happen they are just trying to point out if you want to do the most recent content you are going to struggle without a group who are dedicated to doing the content. so while some groups will be less hard core than others they will still take the people who turn up regularly over someone who turns up once every few weeks, as it would be unfair to the people who put the time in and it would set the group back every time they had to gear up a new player.
I understand people wanting to see all the content and when I started with PCG I never though I would see much raid content but the only way I could get to see it was stick at it and turn up on a regular basis as without putting in the time to gear up and prepare I would never of got see as much as I did with the 25man heroics, the same can't be said for team super, due to it being more casual and having a constant turnover of players it never even got to see the LK. so this really is a demonstration of how much difference having people turn up each week/raid can make.
it's just unfortunate that to see the majority of the end game content you do need to put the time in.