Regarding haste:
Yes, it weakens the characters when it's over. However, you can re-cast haste before it ends, refreshing the duration. I personally do that whenever I'm nearing a series of tough fights, and then let it end when the fighting is over.
I just completed MM7, so I'll be playing MM6 now. I think I found the reason why it felt so easy - haste seems to have been fixed. It used to be bugged from what I know. When all the GM skills (low recovery time etc) are multiplied by GM Light casting Hour of Power, you get a redicilous speed. I was hitting and shooting like a machine gun near the end, decimating everything (including druids) within a few seconds.
It was significantly easier than a four-dragon party in MM8, and that says a lot...