Lots of debate in this one and also lots of room for interpretation (unless Phil himself finds this thread and details his strategy to us all
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It is my observation that Phil is getting a few things with this strategy (for better or worse)...
1. Hellmuth is a tighter player than Negreanu is, so this helps disguise his holdings versus him. If Daniel sometimes raises and others not, then we can try to interpret hand strength somewhat (even if Daniel is raising a mixed/balanced range which I'm sure he is in GTO-fashion). However, by Hellmuth seldom raising at all, he puts himself on a wider range of hands he could possibly hold. Perhaps this decision is because his tighter image would have him raising with a stricter range than Daniel would and therefore reveal his holdings more. (if a LAG open-raises then they could still have a ton in their range, but a NIT like Hellmuth might as well have a neon sign saying AA if he seldom raises)
2. Negreanu is famous for popularizing his "smallball strategy" (which he did not invent by the way). In Negreanu's "small ball", he himself tends to raise small amounts preflop to pursue the blinds and antes; this is balanced by Negreanu limping with some bigger pairs and stronger holdings to help offset his open-raising, so it can't as easily be exploited.
If Negreanu is playing similar to this, then Hellmuth doesn't really gain much by raising. If Hellmuth open-raised 3x or more, then Negreanu calls...now what? Hellmuth has built the pot, sure, but this doesn't say much about Negreanu's hand. Perhaps it is something to attempt a blind steal (like the "small ball" 87s or something) or perhaps it is a trapping hand like KK. Negreanu is rarely folding, so continuing doesn't tell us a great deal about the hand; he could still reasonable float, continue barreling or perhaps have had a premium hand from the start!
3. Player history perhaps? These two have encountered each other at the tables a ton. Maybe a more straight-forward strategy wouldn't be as effective since they both know each other so well as poker players.
These three points are just some of my thoughts off the top of my head, but these are all potential reasons why Hellmuth is playing in this way.
True, he might be missing out on some potential value preflop, but maybe the pros outweigh the cons here? We must assess what this strategy gains as well as what it gives up