Syndra is an immobile champion, so having the dash from Hextech Rocketbelt does help when looking to escape a bad situation (fighting assassins or dealing with enemies who can gap close on you after Scatter the Weak has been used). This dash can also be helpful when looking to close a gap on a low-health target who you otherwise may not have killed with Luden's Tempest

A way to ensure she has enough mana with Hextech Rocketbelt would be to build Tear of the Goddess and stack mana items where possible.

Yes, in the grand scheme Luden's Tempest will offer better stats when looking to build glass cannon but remember, health can be helpful for her as an extra survival stat.