It wasn't add-on after add-on after add-on. Every single part of that Mecha was a character and was correctly explained and introduced in the show. The thing about them being characters was that even though some combinations were hardly used, you'd still see the mecha taking part in the group discussions and taking part in the quest to take down the Gaiark.
Having so many mecha in this show was understandable and things weren't just left to rot in the corner after it's debut.
(Well, maybe except Go-Roader GT)