Venerable Dreadnoughts as a rule have always been too expensive. At least since 3rd edition. You are halfway to a second Dreadnought with a single Venerable Dreadnought, and if you have the FOC slots a second Dreadnought would be better than one Venerable Dreadnought pretty much always.
Plasma Cannons are wicked awesome at killing MEQ/TEQ in the open or when they arrive from Deep Strike. But they can't hurt Flyers or AV 14, they're overkill for your typical horde-lings, and they can't cause Instant Death to T4 critters. So they tend to lose out to the slightly cheaper Missile Launchers which are better against a variety of targets.
That said, when I take Bjorn I give him a Plasma. Not overheating and limited scatter is nice. But there are only two lists I've ever seen where it was okay to take Bjorn competitively, and one of them doesn't work in 6E. On regular Dreadnoughts I'd stick with Autocannons, Assault Cannons, and Multi-Meltas.
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