Without TB Stramie can try to freeze you, and then you have the annoying Lapras and Articuno. Tbolt is actually better than ST for Amnesiers. If you want ST use it in Alakazam. ST in Chansey is only useful for stall wars. But here Reflect is a lot better, and even Flash too. With 8 PP on Blizzard you will only manage to beat Zams only abaut 5% of the time before you get run out of Blizzard PP. If he isn't paralyzed he will obviusly use recover every time it needs. If he isn't para, you will have more chances of beating him with Ice Beam (freeze, more PP). Ice Beam is a lot better for another Chansey. Not to mention that the PP difference can make you the loser in many stall wars. And although Blizzard can 3HKO Tauros, it can miss.