1. Novice quests should be accessible to novices.
It makes sense for players to do low-level quests when they're low-level. The vast, vast majority of players begin playing the free game. They don't subscribe off the bat. By making low-level content accessible to free players, you make it more likely that players will encounter it at the correct level to appreciate it. This is a good thing, yes?
2. It means putting the game's best foot forward.
RuneScape's gameplay is nothing special for an MMORPG. However, one of the areas where it shines is in quests. The quests in this game are pretty damn good compared to its competitors. Song from the Depths and One Piercing Note are pretty much par for the course as far as modern member quests go--quests these days are pretty good--but they're head-and-shoulders above most of the existing F2P quests, which are all, like, ancient relics from way back when the game first came out. Exemplary content like this deserves exposure.
3. A higher-quality free game attracts more members.
Players don't join the members' game because they hate the free game. They subscribe because they like the free game, and they want to play the expanded version. If they're having more fun in F2P, they'll be more likely to upgrade to the full game.
Additionally, a higher-quality free game will attract more new players to the game and help to retain them. Growing the playerbase is obviously a good thing--it means more potential subscribers, a larger market share, all that good stuff.
4. The rewards are exciting for free players, but useless for members.
One Piercing Note rewards you with robes that restore your prayer points when you rest at a musician. Except that that's useless for members--the members' world has prayer potions and energy potions that make musicians largely useless, as well as way more teleports and way more altars. Restoring your prayer at a musician is irrelevant for a member.
Free players, on the other hand, don't have potions to do this stuff for them. Restoring prayer points without needing an altar is much more exciting and relevant in F2P. Free players would eat that [cabbage] up.
Song from the Depths gives you a coral crossbow. Well what are you supposed to do with that piece of junk? It's weaker than an iron crossbow and it can only fire untradable bolts that are weaker than iron bolts. That's crap.
Free players, on the other hand, have a dearth of ranged weapons. The coral crossbow would be the only other crossbow in F2P besides the regular wooden crossbow, making it the only real way for F2P to wear a shield while ranging. And with no level requirements to wield it, it could even be an actual relevant weapon.
Half the Grotworm cave could be blocked off if necessary. Whatever. :mellow:
5. It gives more screentime to the signature heroes.
The signature heroes are supposed to be the face of the game, right? They should show up in the game. Song from the Depths could introduce free players to The Raptor much as The Blood Pact introduced free players to Xenia.
6. It's even a teaser for member content!
Song from the Depths has that whole big thing with the Queen Black Dragon, which you can only fight as a member. And One Piercing Note doesn't have anything blatant like that, but it does introduce you to a man who hunts Desert Strykewyrms, so that sort of counts. So there you go, right? Leave them wanting more. It should at least be something to help sell the idea to the investors.