Edit: this is not using formal statistics in game but rather educated guesses towards showing the benefits vs cost of your time, you all complain that I dont use real numbers but in reality you dont need real numbers to break something down to its statistical nature. im sorry that you cant understand it, the only numbers that im purely guessing is the drop rate for legendary engrams, you can still get legendaries from blue engrams too but at a tiny % chance... dont hate on me for trying to show you guys how you are spending your time relying on luck to the n'th degree to get geared.
Us old timers who appreciate games for what they are will never understand the younger "gotta win everything no matter what" who can only see the lootcave for what it is, gambling their game time; relying on RNG and then another layer of RNG and then more in order to get legendaries/exotics. The math speaks for itself.
lets say 1/300 chance that a legendary engram drops (I'm not sure if that is true or not, its just a guess) and then that legendary engram has a 15/100 (15%) chance to yield a legendary and 1/100 (1%) chance to yield an exotic. that means to get a legendary engram that actually yields a legendary you would have to kill about 1000-2000 enemies depending on other factors that we may not know about (for example if cryptarch rep increases the % to receive legendary or better gear from purple engrams).
the real kicker is when you consider the possibility of receiving an exotic from solely farming... 20,000 - 30,000 enemies killed and again that's excluding all other factors, and probably even being slightly favorable. I'm not a mathematician and have no idea whether this is completely right or wrong but it gives an accurate representation of the reality of farming.
and you could break it down further into time management, lets say sitting at that particular cave you can farm 3000 enemies/hour so to receive an exotic piece (unless weapons it goes even further into RNG with a 1/3 chance for your particular class) it can take anywhere between 5-10 hours of doing this to get an exotic piece of gear (keep in mind 3/8 chance for weapon, 5/8 chance for gear, then 1/5 for which piece of gear, and then 1/3 each for whether or not it will be usable by your class), so modestly to receive an exotic/legendary by solely farming it can take anywheres between 2-10 hours for a legendary gear that is useful and 15-45 hours to farm an exotic piece that is useful.
and one final thing, the reputation farm time, so only and only if you get legendaries from grinding reputation. on a good day i can farm maybe 1500-2000 reputation from farming for lets say 3 hours (collecting materials to sell to the vendors for rep/marks/glimmer/xp FOR EITHER FACTION! and doing patrol missions). this does not include rep from bounties which increase the speed of reputation gain enormously (an additional 250-500 rep depending on the day). so to reach rank two it takes 3500 reputation, to reach rank three it takes 6000 rep. so to start getting guaranteed legendary gear I spend about 4-5 hours, to start getting guaranteed legendary weapons, about 7-10 hours...
After examining these facts then i think it is safe to say these kids farming the loot cave are simply doing nothing more than wasting their time, but coincidentally we are all wasting our time on a video game anyways so I would just prefer wasting mine in a more productive way.
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As much as I wish it was a guaranteed legendary from gaining a rep level, it certainly isn't. I received a whopping 1 ascendant energy for hitting rank 5 with vanguard last night....