Soft control, like slow or prone, is really unexciting on its own, but in the right situations can be tantamount to stunning or immobilizing creatures. Especially at low levels, roughly half of the most common monsters lack ranged attacks, so the real trick is identifying those monsters, beating them on initiative, and never letting them get into the fray in the first place.
Once monsters are next to your allies, yes, slowing them is usually pointless.
Remember, though, that melee allies love prone enemies, especially early on when they don't have auto-CA built into their builds yet. If one or more party members takes world serpent's grasp, slow mobs they're about to hit. This especially combos well with defenders, who you should consider your melee control counterpart.
And if you combo with your allies to knock things prone constantly, any axe or heavy blade users can grab headsman's chop for +5 damage vs prone targets.
Beguiling Strands can be used to disallow enemies ever flanking your front liners, to minion sweep, optimized to push a dozen squares slow and prone, can shove enemies that get in your face right back to your defender, so that your defender doesn't have to break off from his current target to assist you, it can cluster enemies together to prep for an action point and a burst/blast (or just to set them up for an ally's area attack), and it can do all of these things at the same time.
Winged Horde allows allies complete freedom of movement, again preventing them from being flanked and nearly guaranteeing them whatever positioning they desire. It can be enlarged to a 5x5 party friendly area, so is as good as or better than Beguiling Strands for minion sweeping. Every wizard should take one of these two powers, and consider both.
The low damage on both is frustrating, but remember that your job isn't necessarily to do damage. Yes, dead is the best condition to inflict, but you're often going to be more effective setting allies up for success and setting enemies up for failure. The low damage on winged horde will eventually get better, since it gets mods. Beguiling Strands will always be in the 4-10 damage range, but can nonetheless be an extremely versatile, powerful tactical option.