Imagine if you needed to purchase weekly heroic raid unlocks on the BMAH or you can't do heroic modes this week. Everyone would be up in arms about it and the top guilds would be leading the charge. The challenge should be in the content itself, not in begging for invites on /2 for days. If the content will be challenging why add such a massive extra road block to it?
Having said all that there's only so much difficulty they can give these bosses. Ultimately raids are always more complex than 5 mans which are always more complex than solo play. There's just less of a toolset there, not to mention no tank/healer/dps trio.