advice on upgrading mainsheet blocks

[KWKloeber]

What Seldon says maybe true,

In theory yes, but.  "It depends," like all things marine there's nothing absolute and no perfect world - everything's a compromise. 
I'd ask for their test data showing the halyard loads the ball bearings beyond the capabilities.  They have to have that right?  It depends on the loading (size) of the main.  Consider also that Selden doesn't sell reasonably priced ball bearing masthead sheaves, so the next best chance at a sale is to convince a prospect that the another brand is not as good as what Selden's sells?  Even if it's overly conservative.

However Selden DOES SELL high-load bearing sheaves, but they are beyond a typical price point, so...... push the lower-cost ones.

It also depends on whether you are a diehard 4x/week racer or occasional lake cottage sailor.

If GhM offered roller bearing sheaves, it would be the best of both worlds.