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In a monolight, you have the size and weight of the condensers in the head. At some point, that can become an issue.
I find 300-400 ws to be a "sweet spot" in monolights--enough power for most uses and from that point they can usually be dialed down low enough for low power uses--without being too large and heavy.
Obviously, if you frequently need higher power, go for it, but on a practical basis, if you need more than 300-400 ws per head, you're probably in the market for a power pack system.