Blast Buggy is like TetraBoy, which is the appeal it has. If anything DekaWingRobo is the redundant mecha. If it had been a single piece mecha or something besides just another five-piece robo, that would be one thing. But just because the team gets a power-up doesn't mean they need a new mecha to go with it. Look at Gingaman.
Huh... Are you talking from a toy based point of view? If so, I really don't care.
The DekaWing Robo had battle footage pretty different from the ones with the Super/Dekaranger Robo, so it was in no way pointless. Who cares if the toys didn't have some special gimmick? And it did have one, with the secondary combination that turned it into a gun.
So what if it flies, that's not exactly novel among Sentai robos.
But the robot flying around to kick missiles out of the air is pretty rare.
A flying Super Robot with two guns that also turns into a giant gun is a pretty original concept, in or out of Sentai. Much more original than Sentai's standard sword robots which were just based on 70s and early 80s anime.
But you could easily write episodes in a different way. Even if the writers had to include all of Bandai's products, that doesn't mean they must write battle scenes in any specific way. Need proof, look at Kiva. Bashaa Fever doesn't get used ever, whereas some things got used only once and forgotten.
Do you really think Kiva is a good example of merchandising tie in? The way various items were used only once and forgotten was just distracting. That was not good writing. If the toys are getting included, they better actually feel like part of the show, not only random commercials during the series that are almost completely ignored afterwards.
And yet their suits were still designed after those Saints, at least the helmets?
The helmet's shape changed when they went to Legend mode, although not too much. They still had to use their basic forms before going Legend, after all.
That still doesn't mean a new mecha was absolutely necessary. Again, look at Gingaman. Or even Fiveman if you want to go farther back. (Hell even Changeman and Flashman, which didn't come with extra anything.) Your explanation could easily be quoted by a Bandai or Toei spokesman (Don't give them ideas >>).
If the warriors need a power up to face the monsters, wouldn't the robot which faces them as a giant need a power up too? In Changeman, a few times the robot was directly shown on screen to be powered up by the Changeman themselves. So their growth would be reflected on the robot's power.
In Flashman, the FlashKing wasn't really doing too well later in the series, considering how it was trashed twice. Yeah, the Great Titan wasn't directly connected to any power up, but it's basically the same concept. A stronger robot introduced midway through the series.