1) I would suppose that the issue is if you take people who can't solo a tower, they take up the party slot of someone who could. Now, given the nature of wiz-king, you could take up to 3 pikers and not affect the completion time particularly. But, if I was leading a pug, I'd make sure I had the 3 solo capable toons first, lest I get stuck in a group full of people who can't, leading to inflated completion times.
2) Depending on the toon I suppose, but, no, if you can't solo a tower, you won't speed it up. For example, on a cleric life, I won't stop running the entire time. No need to. Blade Barrier works better if I keep the mobs moving anyway. All you do is give the mobs extra hp and higher damage. So, in such a situation, taking a any player who can't solo a tower probably has no real impact on the completion time.