That's true boogy
But he was weaker because of something else ,his mind was sick, he was infected with fear. It's just that which makes him weaker. Otherwise he is stronger than before, yes.
Actualy (yes, I'm going to butt in), Mecha Frieza was indeed stronger than before, but he didn't loose to Trunks because he was afraid and it drained him of his powers, no. Trunks became a Super Saiyan sometime ago and has been fighting the androids for sometime, so it's only natural that he got stronger in his Super Saiyan form, making him stronger than Frieza.
And as to why Goku and Trunks had the same power when they met, is because Goku was on a planet where he was training and since he's a "natural" he got as strong as Trunks.
So basicly, Frieza was stronger in his Mecha form than in his initial form, yet Trunks and Goku have gotten even stronger.
On the matter of adding him in, I'm nevtral. I don't see the good points, since his attacks are all the same except more power and speed and he didn't make such an impact on the series, since the series was bound to end at the end of the Frieza saga.