I see nobody noted an error in the topic maker's 'simple math'. 120 power in two turns is not 60 each turn. Well, first of all, in a practice, 60 x 2 is better than 120 (though it's about 1 or 2 HPs lost, so we can skip this 'problem').
But there's also a good side of 2 turn charge instead of hitting 60 twice. Let's find some grass special move with power 60. If you hit Slowbro with it, he'll propably survive and use Slack Off (if his trainer thinks, I might have mistaken the name, but it works like a Recover) making him, most lilkely, survive the second hit. Solarbeam would do the job immidiately.
Of course, it's easier to Protect against it, but saying that Solarbeam is a weak move, proves lack of understanding it's tactical uses. Also, it's immensely superior to Frenzy Plant and such, as it's only downfall can be easily removed.
I always liked my Arcanine to have Solarbeam, Sunny Day and Flash Fire making him immune to fire and resistant to water moves after using Sunny Day. It' kinda fun to beat Misty with a fire Pokemon OHKO'ing her whole team (sometimes taking two turns to beat Golduck, as he might have Cloud Nine) :D