Sure, you can make gains by increasing reps and/or decreasing rest between sets. But not noticeable gains. Think of all the bodybuilders out there who juice, eat a ton, and train HARD... But they never look any different. They might gain a few lbs A year, but that is completely unnoticeable to anyone but their own eyes. If you want to make real gains, the kind of gains where everyone tells you that you look bigger, the kind of gains that are noticeable even in a sweater and jacket...then you need to lift more weight, not just do more reps and decrease rest between sets. Now, if you take your max weight,and increase reps to 8, then you will see great gains. But when your talking about using a weight you can rep 10 or more times, and using that weight for months on end, just trying to get more reps and decrease rest between sets, then you aren't going to be growing, you'll just be maintaining what you've already got.