I honestly don't see any reason for most bronies to hate Spongebob, but I do agree, they shouldn't really be comparing it to MLP.
Spongebob was actually my favorite cartoon until I was in 4th grade, and I still like it, but I see no reason to compare it to MLP (even though about 6 months ago I was comparing it with extreme prejudice).
Still, my point is I agree with the point that MLP and Spongebob aren't really worth comparing.