There is evidence that probiotics are beneficial to the health of the gastrointestinal system. Dannon Activia was the fist yogurt with added probiotics. Studies showed that probiotics added to Activia improved transit time in the digestive tract.
In addition to Activia, there are other yogurts on the market that also contain probiotics. Those products were not studied though. However, that does not mean that they are not effective. There is just no scientific evidence to support this claim.
From my understanding the short-answer to your question is "yes & no." but do let me elaborate. Though, all kinds of yogurt do contain probiotics, they are not all created equal. There are several kinds of probiotics and some brands of yogurt will only contain 1 or 2 of them...and sometimes in varying amounts.
Those labeled "Probiotic Yogurt" probably add a 3rd kind of probiotic into the mix (usually Lactobacillus acidophilus) and may contain more probiotics than the generic, run-of-the-mill brands. So in that sense, it is not necessarily a "marketing tool".
But, by labeling their yogurt as a "probiotic yogurt", companies are indeed capitalizing on the somewhat recent surge of the health movement. When a health-conscious customer steps in front of the yogurt aisle and is given a choice between "yogurt" & "probiotic yogurt", more than likely they will opt for the latter. In this sense, companies are marketing themselves to the growing number of healthy shoppers. Moreover, for those that are unaware or don't necessarily know what a "probiotic" is, seeing it on their yogurt may spark interest or at least raise the question as to what it is. Once they realize, the health benefits of probiotics, when faced with the decision to choose regular yogurt or the probiotic version, they might as well opt for the healthier version right?
My suggestion: Ditch the mainstream brands/varieties of yogurt and go for Greek-Style Yogurt (some good options include Stonyfield Farm's Oikos brand and Chobani). While most major-brand yogurts contain only one or two cultures of probiotics, these two brands have six. Greek yogurt is also lower in sugar and has higher protein content, which, among other things, makes you feel fuller. Most major brands of yogurt also use sugary additives to sweeten their product...which also make it less healthy. Instead, go for Greek-Style Yogurt & add your favorite type of fruit to give it an all-natural sweetness (my favorites are blueberries & raspberries).
I apologize if this answer was a little long for your taste and I probably threw in more than you had asked for. But, I did want to give you a full, well-balanced answer. Hope this helps!