I know this post is relatively old, but I thought I'd put in my 2 cents anyway.
Yes, IP rewards are nice, but as many have said, the tribunal is for the benefit of the LoL community.
I would agree with 5 IP or no IP. I just did my first tribunal votes, I didn't even know there was a reward for doing so haha, but I still felt satisfied to know that my vote could pass judgement on some of the players that make matches negative and stressful, which is what it shouldn't be. I understand people want to be rewarded for their time, as nobody in real life works for free, but in this case, it is your choice, you can either take that small time period to judge a few cases and get a little ip boost in your spare time, or you can easily just not do it if you aren't satisfied with the system.