Two slaves with the same server id will replicate successfully, but when they reach the end of the master's binary log, something freaks out and forces them to disconnect. This causes both slaves to reconnect, sync (no data needed), and have the connection die off quickly again. The result of this is rapid connection/disconnection by both slaves driving the load to 1+ on both slaves, and to around .3 on the master even in a completely idle system. This is bad. Therefore, server id collisions are bad.
Joins are fast. Joins should be considered standard practice with a properly normalized database schema. Don't fear the join. It's true that join performance degrades non-linearly as the size of the data set increases. Therefore, it doesn't scale as nicely as single table queries, but it still does scale. It's also true that a bird flies faster without any wings, but only straight down.