Head first base running - slide.
Sliding into a bag head first, puts your most valuable asset, ... (no not that asset) - your head .... in the same location as these things:
An incoming baseball usually traveling at high speed. This same object is very hard, compact, and unforgiving when striking the skull, bone, and soft flesh.
A fielder's glove or mitt. The swipe tag was specifically designed to do two things - first to tag you off the throw, and second, to smack the daylights out of ya for trying in the first place. Now in the lower ranks of the amateur game I'm assuming that this is not the case. But, get higher up in the real hard-nose levels of this sport, don't be surprised if you see base runners get smacked now and then.
Every club that I've been with had this as a standing order:
runner on first, stretches his lead beyond the "moon-pie" - that caved out circle on the infield grass just in front of the bag, gets a throw to the first baseman from any fielder to get him back, mandates a friendly "wap" by the first baseman's mitt on the butt .. the first time. Second time, "wap" on the back or lower part of the arms. Third time, .... fielder's choice, but let him know it.
Fielder's cleats are just inches away. Sliding into a bag usually has the arms stretched out and the fingers reaching. This opens the door to all kinds of potential injuries.
In the amateur game, I would assume that fielders are not skilled or trained to "manage base plays", deliberately designed to avoid step'g on a base runner. After all, being a professional in this sport involves other things besides playing the game itself. So, positioning one's self on a base play not only involves .."ready to be taken out", but also how to avoid "spiking" a base running. Plays at second base come to mind here.
Jamming fingers and wrists under/at the end/ and into the side of a bag.
If the base runner has enough force going into a bag, head first with the arms extended, regardless if the bag is a "break-away" or not, jamming a finger or wrist is serious business.
All in all, sliding head first exposes the skull, face, eyes, mouth, neck, collar bone, shoulders, chest and soft flesh to a lot of potential danger.
I'm not a base running coach, nor have I ever held that position. But, I've seen my fair share of injuries - and bench clearings, because of base runners sliding into a bag head first and the quick tempo of play that centered around that experience. Nothing good has come of it.
Should you not slide into a play head first? I guess that would be a personal choice - to each his/her own. On the other hand, I would suggest working on other techniques if possible, to achieve the same thing on the skins.
Good topic, good question.