First off, I don't really buy the idea that "tactical" voting was such a problem that it needed this locking mechanism to prevent it. Not that it didn't happen (or still doesn't), but that it's really worth worrying about. IMHO, it falls into the same category of problems as "someone else found out the solution from my answer and then re-posted it using different words!"
That said... Vote locking is still useful. Why? Because you wouldn't want someone to go back and retract (or reverse) their votes just because they had a bad day and decided that making a nuisance of themselves was the way to feel better. And yes, people would do this (and have done it).
I'm sure you can imagine this happening on a smaller scale as well, which is harder to detect but no less annoying. A system that discourages griefing is a good thing to have... Although this may not have been the best implementation, at least it's fairly easy to understand.
One more thing: you asked about "tactical" up-voting. If that's not obvious, then imagine a post with two answers, one which is obviously much better than the other one but which are close in score. Now, up-vote the mediocre one, causing it to sort above the other (or at least sort equally, since that'll mean it ends up first on the page randomly until the scores change). Folks might down-vote it, but generally-speaking people are more likely to up-vote than to down-vote... So your answer gets at least one more up-vote this way. Then you remove the up-vote from the mediocre answer and watch it sink away... Why remove it? Because there's always a chance that the author will actually improve his post to the point where it might legitimately compete with your own (or at least seem like a reasonable answer to folks who didn't scroll) - in which case you'd want folks to see yours first, and would need to get that up-vote removed as quickly as possible.