Ok, alternate picking is probably the most useful picking technique, but that doesn´t mean that it´s always the one to go for. Sometimes, the downstroke is the only stroke you´ll need. This may seem unefficient - to only use downstokes requires more strength than if you would use the usual up/down-pattern. But using only downstrokes can add an element to your playing that is impossible to gain otherwise: aggressivity. And this is an important thing in many genres. I´ve used it in thrash metal, death metal, punk and even in highly intensive pop songs.
Listen to the Ramones for an example, a band that used downstrokes as a rule for their riffs. Listen to the intensity to these riffs, listen to the feeling. Their raw qualities wouldn´t be there if Johnny Ramone would have used alternate picking in these riffs.
Another great example is Metallica. You have probably heard a couple of Metallica songs in your life, but have you listened to them and thought about the way the riffs are played? James Hetfield is one of the best downpickers in the business and you can hear this in many of his songs. "Master Of Puppets" is a great example of this. James uses downpicking to get that aggressive feel to the riffs and combines it with palm muting where he needs a bit more chunk.
Even in solos this approach can be effective. You´ll sometimes get a more tough sound to your lines when you diss alternate picking in favour of downstrokes only. Sure, you problably won´t be able to play as fast as you can otherwise, but you´ll gain a tool which can accentuate tones in a really great way.
So, how do you practice this? My best tip is to look into some of your favourite metal songs and look at the picking hand of the rhyhm guitarist. Then, try to play their songs for yourself. Don´t use a floppy pick, you´ll need a pretty heavy one to achieve what you´re after, and keep your eyes on the picking hand. If you start to play alternate picking when you only should use downstrokes, then stop and start over again in a lower tempo. When you´ve nailed it, speed it up a bit and continue.
And for a bit of inspiration, check this out. Even if you´re not into metal, developing a good downpicking strength is good idea for anyone - it can be used in many other ways than just in the more extreme metal styles: