The human gut has about 10 billion bacteria of up to 400-500 various kinds per gram of contents, totaling an average of 4 lbs bacteria per gut. So, without bothering to do the math, we're talkin' about many trillions of bacteria in an intestine. Logically, a probiotic which states "millions" of critters is a joke. Even a "15 billion" capsule might seem modest in comparison to 4 lbs (trillions) of bacteria. I can't see how going way over the prescribed amount could be harmful unless once experiences distress as a result -- after all, these are beneficial bacteria. Then again, you might see no gain from doing so either. I'd say it's mainly a pocketbook issue; taking a half-dozen "15 billion" capsules a day might cost something like $3/day? Most people won't accept that, and I'm sure that's factored into the label's recommended dosage -- they don't want to "dose themselves" out of the market by suggesting a $3 dose/day to potential customers.
I say that more probiotics is better...unless you start to feel distress by doing so...or if you don't realize any benefit by greatly increasing the dose (then what's the point?). I've read in a few books that probiotics with many varieties of bacteria might compete with the bugs effectively cancelling each other out, so that a refrigerated, high-potency product with just one or two critters (typically acidophilus and bifidus) are best.