If I only stuck to shows I knew I would like, I'd be missing out on so many and my favorite character list would be half as big. You can't tell some shows are bad at the start or they will stay bad.
Take Yu Yu Hakusho. Almost nobody talks about the first dozen or so episodes because while they're not bad, aren't anything special and sometimes boring. But it gets pretty damned epic a few dozen episodes in.
There's much to be said about Clannad, Kanon, and Nanoha, since I didn't like their first parts at all either, but I would have been a very different anime viewer if I didn't, most likely for the worse.
I can also keep watching a less than par series if there's something worth watching for in spite of the rest. For example, I think that series like Hana Saku Iroha and Oreimo are incredibly mediocre, if not badly written, series but the likes of Ohana and Kuroneko allow me to bypass such things and enjoy myself while I'm not facepalming at whatever stupid shit the show pulls when they're not on screen.
Finally, there's also shows that provide eye candy and nothing else. This usually applies to mediocre series with fanservice and a good budget.
Also, it's funny to hate sometimes just like the many people that like riffing out there. Watching bad anime once in a while also helps you appreciate good anime. Just don't overdo it because you might become a bitter douchebag, and nobody likes them.