I personally had this same problem so I'll give you my view and final decision on this.
Its always hard when you can't do enough to save yourself or kill the enemy in a single turn, but that's why there are RB pets. For me, I always thought I was just gonna have an extra damage dealer...that turned into 2 damage dealer RB pets and 2 support RB pets.
I have my Earth and Water Wits with Xoalan And Shasha to provide support whenever I need it. They are great meatshields and xoalans provides the stone wall while shasha helps with crowd control. They're also great for using items to heal damaged team members or give my Water Wit the freedom to use revive while they use items to heal other units rather than the water wit having to do so.
For my fire killer and fire knight, I have my Fred and Niss be damage dealers next to my fire killer. It works out perfectly like this:
Turn 1: Fire Knight HF niss, Fire Killer HF herself, Niss and Fred attack
Turn 2: Fire Knight HF Fred, Fire Killer Attacks with niss and fred
Turn 3: Fire Knight supports by either attacking or using items or JS, Everyone else attacks.
Turn 4: repeat turn 1 and so on...
This setup makes it so that I have a turn to support my units by either healing or using a support skill to help the team survive. It makes getting through battles faster and provides more survivability.
Of course, the ultimate downside is the cost to make these RB pets and maintain them. If you're rich or have a lot of material characters to provide you with food and healing materials, then battle costs should be fine. But when it comes to growing these units and making armor for them...that's a whole different scenario. It takes time and money to grow an RB pet like my Niss and Fred from lv 1 to 100 by regular training. What's more, Bursting for my Xoalan and Shasha costed quite a bit too. Whats more, I always want to provide the best armor for my RB pets since I need them to have good stats and survive with my main characters so my Alchemist has to work her ass off trying to make MATK equipment for niss and fred and DEF and SPD items for xoalan and shasha.
That's my take on it atleast. In the end, it's worth it to me since now I have the strength to finish most mission without having to log out and enter the battle again. So yah, hopefully someone who only takes one RB pet will give you their view so you have both sides to consider.
