On the latest Shunners and Sinners podcast, episode 5, the discussion (and there was a lot of brilliant discussion in this long episode, highly recommended) turned to low sec and co-host Kaspa101 asked "What is the point of low sec?". With the recent changes to the Crimewatch system and gate guns having less of an impact on fights at gates, he feels that there is no distinguishing or redeeming feature of low sec and it should just be converted into an extension of NPC null sec.
I think his opinion comes from not actually living and fighting in low sec full time as he is a long time null sec resident, and its an opinion that I would have shared a couple years ago when I was a full time null sec pilot. But after living in low sec for a long while now, I can confidently say that low sec brings a very divergent and unique battlefield to the game that other parts do not.
First off, we have the restrictions on certain weapons. No stealth bomber bombs, warp interdiction bubbles, heavy interdiction bubbles, and Titan doomsdays. Its a small list, but significant. The first three are area of effect weapons that are more deadly the smaller the group you are in. The restriction on these weapons allows smaller fleet sizes to operate without undue repercussions. The restriction on the dooms day device has less impact on sub cap fleets but does mean you are less likely to see titans in low sec as dreadnoughts are far more cost efficient (unless, of course, you have issues distinguishing between Bridge and Jump).
Secondly, the proximity to high sec coupled with NPC stations means that the population tends to be more varied with the occasional high sec dweller attempting to brave the dangerous lanes for higher rewards. They usually quickly learn the risk is higher than the reward, but at least you see them more than in null sec. Admittedly, this feature might not go away if you make low sec into NPC null sec as it will still be adjacent to high sec, but the psychological value of low sec NOT being null sec probably entices more attempts than if it had been just null sec to begin with.
Combine these two and you get a unique battlefield for pilots to traverse that is not present in the other zones (being Wormhole space, NPC Null Sec, Null Sec, and High Sec) and a greater variety of combat zones (for lack of a better word) means that pilots have a greater chance of finding a part of space with combat that suits them.
Summary: The point of low sec is to have a combat area that is less restrictive than high sec but more restrictive than null sec such that it is inviting to those interested in trying out PvP but nervous about dealing with warp interdiction spheres and bombs and doomsdays.