While I love Land Speeder Storms as much as the next guy, I'd like to point out that the fact that you were able to fire your Multi-Melta means that your Speeder moved no more than 12 inches.
What if, for instance, your Multi-Melta shot fails to destroy said transport? The scouts would either have to sit in their Speeder (which has no cover save), and take return fire on your enemy's turn, or assault the transport itself and hope that the Powerfist will be able to hit the transport. Even then, if they manage to destroy the transport through assault (pretty unlikely, but it happens) they won't be locked in the relative safety of assault. Meaning they're going to get shot.
Honestly, I'd rather use my Land Speeder Storms to deliver my Scouts to things like Long Fangs, Broadside Battlesuits, etc.