I don't think it's very narrow at all. I read about the first 10 issues of Journey Into Mystery (when it all rebooted during the laughable Fear Itself) and I had to drop it because of Kid Loki. I'll admit Leah was cool because of her antagonistic nature towards KL, but it wasn't nearly enough to warrant a monthly buy. If it takes more than ten issues of a book for a character to show something new then I will drop the book. That being said, it was during Fear Itself and that was a poorly plotted and delivered event. So my dismissal of the book (and subsequently, the character) could be skewed by that.
I mean maybe this is just my strange taste showing or something, since I honestly haven't met very many people who actually read that whole arc. Although of the people I know and the ones I've seen commenting on it online who did read the whole thing most everyone seems to be a fairly dedicated fan. I've seen some really awesome cosplays of Loki and Leah too from fans, and I'd personally love to do Leah but I'd have to find someone to do Loki, haha.
The book did have some rad covers, though.
The covers were really awesome, and all of the issues that Stephanie Hans illustrated and colored were gorgeous as well, I hope she gets to work on more Marvel stuff in the future.