When I first bought it on PSN I thought it was kind of wierd too. The gameplay reminds me of the multiplayer Siege mode in Ratchet and Clank 3. I am sure you know about that since one of your favorite Ratchet and Clank games is that. The game has the upgradable weapons if that wasn't in the demo and the campaign mode is like doing a normal ratchet and clank mission while still coming back to protect your base. Its definetly not my favorite ratchet and Clank game, but it is not the worst. (Not counting Size Matters or Secret agent clank which are the worst ones in m opinion)
The Ratchet series has always thrown in new elements to change things up, I think Insomniac tries to keep things fresh that way. Sometimes they fail but I think they succeeded with Full Frontal Assault. Try to see FFA for what it is and not for what it isn't. Look at it in the same way you were able to appreciate Deadlocked after you had some time away from it (and from Up Your Arsenal).
I like the Looney Tunes cartoons from the 40's and 50's but I can also appreciate the new Looney Tunes Show. It's totally different, features the same characters we've loved for years and it's still funny. I take the same attitude with comparing Full Frontal Assault to the earlier games.
Also: campaign mode in Full Frontal Assault is better with 2 players, I'm going online now to drum up a multiplayer game. I'm BR4CR on PSN if you're looking to play some co-op or competitive online.