I'm not a big fan of 7th through 9th level spells. I like the old spell levels of 1 to 6. It keeps magic somewhat under control.
This doesn't mean I'm against ever using those high-level spells. Let magic-users learn 7th level spells and higher, but without the ability to prepare or memorize them. They have to spend time and money to inscribe a scroll and cast it, with a chance of spell failure or fumble, exactly as if they were casting a spell too high for them to cast. Add the MU's HD or half their level to their d6 roll. Likewise, I wouldn't let clerics cast 6th or 7th level spells on an adventure; these are miracles that must be prayed for at a shrine or temple, and they directly contact the deity for assistance.
Both options give high-level spells more of a ritualistic and risky feel.