Unless the hub is inappropriate or written with poor grammar, I always leave a comment. This is the least I can do when a fellow hubber has taken the time to write a hub that I respond to.
Comments can be helpful not only to the writer but to others who comment. There have been times when the actual comment has been more interesting and useful then the hub itself.
We are a "community" - a "family" and it's important to support, motivate and inspire one another. How else can we do this except by commenting?
The word is the most powerful of all sources. And with our words (comments) we have the power to change someone's entire day and out-look.
Sure, it takes time to write a few sincere words of appreciation. But what better way to spend our time than to be a support system to someone who needs it.
If I had not received the comments left on my first few hubs 2 years ago, I wouldn't be here now. I was scared and unsure of myself. I was just a little "ripple" among a sea of giant waves (great writers.)
I will continue to comment on hubs. And who knows? Maybe I am a bigger wave now with a chance to help another little ripple here on hubpages.