What about Spike?
Well Spike can't run around with his lvl 2, Parappa can.
BUT Parappa's lvl 2 can be dodged, easily and if he uses too much time on one person, then the rest of the guys in a FFA will be free. So a lvl 3 where he gets 3 kills instantly is fine.
Screen clearers aren't useless. They can be used tactically to nullify active supers. Say Heihachi just brought out Kuma, you can activate the level 3 to remove Kuma and score yourself some kills in the process. It's all about risk/reward.
Ya screen clears arent useless in any way. They are just another part of the strategy in 1Vs1. Considering that Kuma, the skateboard, and Spike's slash can all be easily dodged in 1v1, and their level 3 only take a few more hits to get to, it is almost always worth it to go to level 3. In FFA's, its not quite as important, but still it has its uses. The 1v1 game has a surprising amount of metagame for someone like heihachi because he has a screen clear. just part of the fun.