They fill a narrative void that had been lacking for some time--that of heroes, like our player character, who are trying to make their way in the world. We never directly meet any other actual players in quests (no, the Black Arm/Phoenix Gang parts don't count), so we need stand-ins that feel like they're just as important as us. The Legends Guild and other "adventurer congregations" have always felt very empty--and with the Signature Heroes we get to see some of the other members.
The major issue is, like the mods discussed in the podcast, that we haven't seen much of them yet. There has only been three quests (counting the two-parters as single quests) and a Fremmenik Saga that have featured them at all so far (mostly because the JMods really don't want to shoehorn them into quests)--they feel very one-dimensional and basic because of that. They haven't had the time to develop into likable, relatable characters like those we have come to expect. If they had never pointed them out as "SIgnature Heroes", there would never have been any specific hatred pointed towards them.
Do you remember Cyrisus? Any of the heroes who died in WGS? More recently, the PoP adventurers? And what about other players? In Runescape, other players play the other adventurers, not NPCs. We are supposed to be reasonably unique, certainly at the level of power we represent with TWW done. There is no need for NPC adventurers to fill out the narrative, that's where co-op quests come in (would be awesome, dg-style questing).
The main problem with signature heroes is that they are set up as marketing scheme. They are extremely, extremely
heavy on cliches - the good knight, the charming rogue, the single-minded character (in this case a blacksmith - she practically denies the usefulness of anything besides metal weapons, to the point that it feels very forced), the curious/investigative character (also happens to be the wizard in the standard party of four, and the 'hot girl'). Plus a stock mysterious badass character. Annoyingly, they're all so bloody nice as well. Sure, Ariane and rogue dude have been 'cast out' but that was clearly
for all the wrong reasons. Cyrisus at least had some real issues - he was bloody useless in combat as a consequence.
Third point: the signature heroes are nothing
like players - we are all-rounders, they are specialists. We all have access to all the powers, they use only their unique powers. We use crossbow shortcuts where they are built into the game, they use (cross)bow shortcuts only where quests call for them. Again, Cyrisus at least used the equipment that was high-end at the time - his bank was really good, but it was a player's bank, not filled with Ariane's Unique Hairpins now available for just $8.99 (order here