dont cheap out on hard armor. it may work but unassing any bad situation in 15lbs of plates will wear on you fast..
Strongly agree -- steel armor might work for say armor of last resort like those SKSes or PSA ARs some folks keep for anyone w/o (as armor for those folks you're giving cheapo but hopefully tested and proven reliable guns to). But definitely get some 3+ multicurve synthetic plates in SAPI (or better yet shooters cut) and run with those on the regular.
Beyond above, the only places steel plates make sense for me are a set or two to keep in your car (heat won't hurt and can mount in or outside vehicle front or behind or side you if you need to move through trouble and already have your lighter everyday armor on) AND use for permanently fortifying hard defensive positions in your home like under or beside windows, your bedroom hallway/doorway(s), etc. Anywhere with moisture, you're ahead to use sand, dirt, stone, timbers, etc.
I also think having a least one level 4 plate (non-steel) available is a good idea per member of your tribe if you have the budget, but quality and tested 4+ plates run ~10+ lbs so like steel get heavy fast. And if you're encountering opposing forces with armor piercing projectiles (your 3+ should cover M855, so most scenarios) where you know you need level 4 armor, you might be better served to just withdraw or not enter the fight in the first place... And a 3+ plate combined with a 3a or 3a+ soft armor backer should provide pretty close to level 4 protection and give you some additional margin of error if shot -- personally, large SAPI vests fit me best, which can run 11x14 plates, but I find 10x12 plates with 11x14 soft armor is best balance between protection and comfort for regular wear.
And before level 4 plates, I'd strongly recommend looking into night vision and flir, improved muzzle devices or suppressors that mitigate flash, small unit and ambush tactics training (above and beyond standard trigger pulling classes), quality camo that's NOT the crappy hunting stuff that's too dark to actually work (natural gear is one of few w/o that issue -- and new Kryptec Obscura Transitional looks promising as well -- and not really a multicam fan but it's more effective than most commercial hunting camos) etc, that make it MUCH less likely that you'll get shot in the first place. Also look at adding abdominal armor, armored helmets, etc, before you get into level 4 plates.