Why is cracked bath tile a problem?
I am going to guess from the look of the tile that the shower was installed years ago.  It was probably installed directly on some form of drywall.  Using drywall vs concrete board has one dramatic affect (other than waterproofing).  It will move/swell more during hot/cold, humid/nonhumid seasons.  

Is it a huge concern?  Well at the very least I would epoxy the cracks.  Normally I would replace offending tiles.  If you have access behind this wall, what do you see?  Is it drywall?  If so that kind of answers your question.  Does it need to be replaced right away.  No.  Especially if you can see it.  Also since you have access to the wall from behind, how solid is the wall?  There should basically be zero movement on a bathroom wall.

You also might want to think about adding more insulation to your house.  If temperature and humidity changes are varying enough to cause cracking, maybe you can solve the root cause.  Then maybe the cracked tile lasts long enough to where you can just replace it when you want.