No, that's not true at all. Strength does benefit your weapon damage, because weapon damage is based off the weapons stats and a coefficient of your strength depending on what weapon type it is (1 hand, 2 hand). You can look it up on wowpedia.
However, I believe last I checked (might be wrong it was a long time ago) the strength coefficient for Mortal Strike (which is normalized at 3.3 or 3.4 or whatever) is only 20%. How that even translates into damage, I have no idea.
That said, Slam is not normalized, which means it uses the full might of BAR's 3.8 weapon speed, meaning strength should scale harder with Slam than it does with Mortal Strike. But how much that is, and whether or not it's worth sacrificing strength for critical is another matter.
=) I'm really looking forward to getting my BAR this weekend. Didn't even realize you can get 160+ tickets if you get every artifact + do all the professions + first time 10 ticket bonus.