How valuable are poster presentations for new Ph.D. student?
It depends on how well the poster session is organized, and on the culture in your discipline (do people show up, do they engage with the people next to the poster or just do the pro forma lap past all posters and leave again, etc.). So there is no single answer to this question. 

However, I come from a discipline that puts little value on poster presentations, and I disagree. For me the poster presentations are the ones that have been most productive. In a poster session there is much more room for me to interact with the participants, while in a presentation it is me talking followed by one or two questions. It is the interaction (questions) from the audience that tells me if my argument is clear, if I made any mistakes, if there are some interesting angles I have missed, if there is another group that does interesting complementary work, etc. etc. So I have received a lot more interesting feedback from poster presentation than from oral presentations. Also, if a part of my argument is unclear, it typically takes a bit of back and forth to pin down where the exact problem is. In a oral presentation there is typically little room for that, but such a conversation is perfectly natural for a poster presentation.

So my suggestion is to ask someone who has attended that conference before whether the poster session was half-way decent (not 7 in the morning on the day after the conference diner, 20 minutes walk from the main venue, parallel to lunch which was served on another location, or other disasters). If there are no disasters, then a poster presentation is probably well worth it.