You know, I have had my 661 now for almost a year and a half, and not once have I found using NiMH internal AAs to be a limitation. I have had the unit turned on for close to 5 hours several times without even getting close to low power indication (line in).
For me, externals are just another thing to carry that with this recorder, are unecessary with the possible exception of an all day festival. I can still see just 2 sets of AAs being enough for that too. Just my opinion, everyone has their own needs of course.