When the job market is in boom it really dosent matter how many companies you hop, but when the market will slow down a little bit and when companies become selective then it will definitely have some bad effects.
I feel it is all matter of time ,If it is boom time does not matter but in the slow down HR might be sceptical to hire such candidates. But in the long run job hopping in every six months would not yield any significant impact because there is limit for pay hike ,after that it would be very difficult to get hike. Nowadays there is such a huge gap between demand and supply that nobody cares how many job person has changed,so when the picutre is not that rosy person has to be careful. the problem is that nowadays everybody wants to mint money as soon as possible like there is no tomorrow.Things would improve with the time when there would not be such gap between demand and supply. By the way companies are also partly responsible for such things.
Eventually the market will slow down. Even when we're not in boom or bust cycles, the market will increase and decrease naturally. When the market decreases, employers are going to spot job-hopping behavior and that candidate will have a much harder time trying to get a position.
Furthermore, in more selective companies, regardless of the economy, they're not going to allow themselves to be used as someone's stepping stone.
I've also noticed that it doesn't do well for a long-term career path. It's very difficult to gain seniority or demonstrate stability when you're job hopping. I've seen people stick with a company and demonstrate themselves as a good candidate for management. The job hoppers are now struggling to sell themselves because their core skills are becoming increasingly out-of-date and they're now chasing the technology.
In short, I'm not against switching jobs if there is some sort of long-term gain. However, it's not a good practice in general because the short-term gain is unlikely to balance out the long-term harm.