Although it is possible that I'm mistaken, I think that the reason that they are separate is so that guilds have the option of sending them to three different tiers. (Even if that isn't the reason, I know that my guild uses them this way - since we have players with crafters at varying levels that we support in this manner - and I have a hard time believing that we are the only ones in the entire game to do so. )
The pony and goblin both harvest everything from a given tier, plus the goblin has a chance at rares. I send each of mine out for a different tier. It seems strange that the hirelings I actually pay coin and status upkeep for every week do less than my pony and goblin. I'd love to see them consolidated into a single amenity. Maybe they could allow up to three per guild (like the mailbox amenity works) so that guilds could harvest multiple tiers if they wanted to pay the upkeep on having multiples.
Ostensibly, the hirelings are collecting resources for the entire guild, not just one person (although, granted, a guild can consist of just one person). But, as designed, the harvesting hirelings are meant for use by multiple people. (That isn't saying that you can't use the resources for just one person, just that they are intended for more than one.) But don't take my word for it. PM Kaitheel and ask him if the guild hall harvesting hirelings are primarily intended for individual or multiple person use. On the other hand/paw/claw, the pony and goblin are intended for individual use. I would support there being an option to consolidate them and to re-separate them - if guilds that wanted to do either were willing to pay a minor nominal fee. This would allow guilds that want to combine them to do so, without affecting those that don't.
Being able to send the three different NPCS out for three different tiers of materials is very nice. The amenties were in the game well before the pony existed and it's something that anybody/everybody no matter what TS level or harvesting skill can use. They have a number of benefits over the ponies.
Would be nice if you had the option to have them harvest only one item, such as only bushes. A few harvests, such as some little used ores, may have hit the limit of 99K or so, while you might still want other harvests of the same tier.
This is one of the reasons having them separate is nice... you don't have to get the extra metals. Just don't send that harvester out at a tier you've filled up.
