There are a number of different scenarios that will grant extra dice. Such as attacking from range 1. There's even a pilot ability in the core set that adds an extra attack dice from attacking at range 1. In theory the core set could have a scenario where the attacker is granted 5 attack dice. If I roll 3 hits do I just remember that I've already got 3 hits and roll 2 more dice? Is getting another set of dice a common practice?
Additionally some of the upgrade cards it isn't clear if they're single use. The way the text is written it implies that you don't use forever (proton torpedoes) but it doesn't necessarily say discard (R2-D2).
Finally there are some expansions that are out of stock on the website (like the TIE Defender and Y-Wing). Are they out of production or just out of stock?