That's kind of mean. Tangela was actually pretty solid in Gen-1. Definitely a viable play. It was the only pure Grass-type in the game, so it wasn't ludicrously weak to overpowered Psychic types (which exploited the poison weakness in so many peers).
If you got the think leveled-up and taught it...
Sleep Power
Mega Drain
Growth
+somethingAOL.ke Slam
And just did the "go to sleep, drain their health, restore yours" thing with it, you could keep going for awhile if your opponent lacked a fire or flying type around. Definitely did a good job neutralizing the omnipresent water, electric, and psychic types out there.