For a power plant, you need to get more energy out than you put in. The best we've done is output that's about 70% of the input energy.
But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion.
On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.
But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion.
On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.