The additional solo-ability.
I use high rates as a place where I can explore places that i can't normally go to without a lot of complications, or with so much chaos of playing in a team that I can't simply sit back, relax, and enjoy the environment a little. I'm an exploring kind of person, and I like finding all those subtle little places and signs of symbology that a lot of people like to miss. However, in some areas, the combat can be so intense and distracting, that it's so, so much harder to look around.
That's my take, at least. xD
Also, from my point of view - and this may simply be my constant bad experiences - they also have the nice, pleasant, relaxed people. Most level to the max, get maximum gear, so on and so forth, within a week or three, and then relax and enjoy there time talking and playing with the community. They play the game because they have time to kill, or just enjoy it.
I've found so many agitating pricks in low rates, that I've kind of given up going into the community in a "head first" kind of way. The only way I've ever enjoyed low rates is when I hide with a group of awesome people in a guild, and that's only a few people, not the community. There are those nice people in them, mind, but I've only found them doing the same things I do: hide with a group in a guild.
There are those few lucky servers that don't look like this, but there are so many more that do, that I've generally avoided low rates like a plague, unless I know enough people that go to the server to meld in and enjoy my time with those few people enough to inspire me to play.
The in-game play itself is often better, especially when you hit WoE, as it's so much more involving, however, so I always love it from that aspect of low rates. =D
I've got to say though, I agree with the transition point of view a little too much. xD I started playing from the first open beta in iRO, so I know what it means to truly grind. However, as time progressed, I've consistently gotten used to faster, higher rates. Alas, I now have a bit of a hard time getting adjusted to rates lower than 15x. So... Alas, while I'm more of a fan of mid-rates than high, I do accept that a lot of high rate-ish players have quite a bit of problems getting used to low rates again. D=