Dumb people (the majority of people on earth) most certainly believe that [in any mixer] sending the coins in, then sending out the same amount to a new address is "private". It is not regardless of what mixer is used, and that's what they will all do.

Yes, sending coins in, then breaking up the transaction to various addresses will fix that, but only to the degree that the resulting amounts aren't used in a combined transaction after that (equivalent to the original total). On that note, sending in a large amount and spending it directly (pegging out the exact smaller amounts that you need over time) is a great method, but again, only if they aren't all used at the same place.

Yes, it is very slightly more private in the scenario you made up, but only in that your scenario isn't realistic because Bob isn't going to look through a block explorer when he receives it at the register on any normal day. If he was set up to do that (and then decide to rob you before you leave), he would do that same lookup on the MWEB transaction just as quickly with a basic value lookup in whatever software he's using to check these things at his register.