ForensicYOYO said:
I'm not usually concerned with ecchi (or even Hentai for that matter), but there's a time and place for everything. While it can add to the experience in some animé genres or pieces, it'll detract from others, yet seem to fairly common overall even in places where it doesn't belong. For instance, having a gang of water melon smugglers jiggling around in an otherwise serious suspense thriller just isn't the right combination.
See the first episode of Higurashi no Naku Koro ni Rei for a staggering example of a "swimsuit episode" not only severely disturbing in its own right (given that the almost the entire main cast is or looks blatantly underage), but completely and polar opposite to everything that made the first two seasons good (horror/mystery/suspense etc.).
Some of it can also get somewhat too weird and/or fetishistic for a larger (western) audience to enjoy, taking away from the overall experience.
On the other hand, there are genre's and animé where it's entirely appropriate, and Highschool of the Dead would seem such an Animé to me at least from what I've heard of it (though I haven't checked it out yet). Can't really understand the protests on it in that particular regard, but maybe people are just fed up with it due to overexposure in so many Animé titles?
TL;DR:
There's nothing wrong with fanservice in itself, it's just spread out to more genres and shows than it could remain a benefit to. I think much of the resentment comes from that.