役立つ sounds pretty stiff/literary as a simple predicate. The more common expression is 役に立つ. It's not like you drop the に in 役に立つ to get to 役立つ. They are two separate terms, that aren't (always) used the same way. If you're talking about the に you'd usually see in front of 役立つ — I don't think you can drop it in cases such as this one: この辞書は勉強に役立つ. You'll sometimes see it following で, は, or an expression roughly equivalent to に (e.g. において), but that's not really "dropping" it. Just a different structure altogether.
Edit: I forgot to cite sources — [Source]. I'll see if I can find some more.
役立つ sounds pretty stiff/literary as a simple predicate. The more common expression is 役に立つ. It's not like you drop the に in 役に立つ to get to 役立つ. They aren't the same thing. If you're talking about the に you'd usually see in front of 役立つ — I don't think you can drop it in cases such as this one: この辞書は勉強に役立つ. You'll sometimes see it following で, は, or an expression roughly equivalent to に (e.g. において), but that's not really "dropping" it. Just a different structure altogether.
Edit: I Forgot to cite sources — [Source]. I'll see if I can find some more.
PS: Let me know if I misunderstood your question.
Ahh... i see then.. に is important... thought やくだつ would be like a summary of やくにたつ..
