Saying that the Plus is useless if you have an iPad and/or laptop is a spurious argument. I don't bring my tablet and laptop with me everywhere I go. I do bring my phone with me everywhere I go. For places that it's impractical to pull out and use a tablet or laptop, I can use my phone. You can argue that 5.5" is too impractical for a phone (which the proliferation of large screen phones on the market and the high demand for the 6+ seems to disprove), but it is still a phone and as such can be used in situations where a tablet or laptop would be impractical.
Also, different people react differently to these phones. I think the 5S is tiny. The 6 is still too small. The 6+ is a great size for me. It amazes me how much antagonism there's been towards the 6+. Some of it is from people who got a 6+ and regretted it, and that's a little understandable. But a lot of it is from people who haven't even touched a 6+, and that makes no sense to me.