It is no news to most girls that have tried internet dating, that men often send out generic messages to large number of girls. I recently gave online dating a shot for a few months, and saw a lot of complaints about this in women's profiles. They would end their self-description with a sentence to the effect of "If you are just going to send me a generic message that is obviously being sent to multiple other girls as well, please do not expect a response! :)" I have also heard girls complain about this in person. While I understand how girls would be less than satisfied with receiving what is essentially junk mail, I also understand why men do it.
First, let me clear up what I think is a common misconception. Women tend to assume that men use mass messaging to get lots of women, in other words, that they are internet "players" and are simply using statistics to get laid. There is some truth to this. In fact I know at least one guy that sends out as many messages as possible for this reason. However, the majority of men do not start with this attitude, and a man's use of mass messaging is not a good indication that he has adopted it either.
Men send out mass messages for a very simple reason: women don't respond frequently to sincere, personalized messages. In fact they almost never respond. I would estimate that in the three months I spent internet dating, my rate of message return was less than 5 %. If you consider messaging to be successful only if it ends in a phone conversation or date, then knock that down to 0 %. And I was messaging a healthy portion of girls that I wouldn't normally consider approaching in person, only because I got no replies on the messages I sent out to girls that I would approach. (As a point of reference, I would say that I have about 60 % success with girls I approach in person; so my failure online surprised me - after all, internet dating was supposedly accessible to anyone, regardless of shyness, etc.) But while my lack of success just caused me to give up on internet dating entirely and focus my time and energy back on meeting girls in person, many men that are faced with this situation persist and simply change tactics: they write a single message, which is generic enough to apply to any girl, and send it to every girl that looks good in her pictures. If they get a response, then they read the girl's profile and decide whether or not to message her back.
Most men don't want to operate this way, but when you are spending all kinds of time reading profiles and writing a personalized messages, only to get a series of "viewed profile" notices and an empty inbox, it is time to give up or change your approach. Mass messaging might not be the right approach, but it happens for a legitimate reason.