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Let me preface this. There is only 1 real reason to lv200 pot a pet, Divine Link, and two main reasons to be using it:
1. Your pet becomes a tank sponge for you. HP/DEF/PROT/MDEF/MPROT are important in this case. You want your pet to survive as long as possible.
2. Your pet is also a stat stick for you. A portion of their stats (HP/MP/STAM/STR/DEX/...) are transferred to you. So a pet with high stats is a nice bonus touch on capping out stats or gaining more max dmg.
This means a Nimbus will only ever have 5/5/5/5 of each stat and 20HP (Not including stats gained from skills) with its only saving grace being 50% Protection. Now you're right that the on-summon effect is nice, but that's irrelevant in the case of Divine Link which will have your pet out before the engagement in 90% of the scenarios (I assume that's what you mean by AoE). If you wanted to maximize protection from Divine Link, you would choose the Phoenix where you wouldn't be sacrificing 500+ stat gains for 50% protection. So while Nimbus does have nice protection, it doesn't have an HP pool to survive with.
Now the reason why Scooter Imp and Fallen Fairy are the two best pets is because they have a good mix between these two reasons. Very high base HP pools with nice stat gains (400 vs 350HP/ 150 vs 100 STR/ etc..). The Scooter Imp, however, makes up for it's lack of stats with "Tiny Jibes" which passively reduces the DEF/PROT of enemies around it. As a result, it's not reliant on "on-summon" effects. Fallen Fairy, on the other hand is an amazing mix of Stats and Tankiness. A lot of people compare Bone Dragon and Fallen Fairy for this reason. People forget that Bone Dragon is a HUGE pet. It takes up large amounts of screen real estate, which is inconvenient (especially if you're a giant). Secondly, it's incredibly slow. Enemies beating on your Bone Dragon will likely kite it away from you which, in the worse case scenario, can break Divine Link. Even when looking at the stat differences between the Fallen Fairy and the Bone Dragon, the differences are negligible at worst and paltry at best (20 STR, and yet the Fallen Fairy still has 1 more Max DMG). Lastly, the other benefits that Bone Dragon provides are outside of Divine Link (On-Summon DEF/PROT break and On-Death weapon power pot).
Sorry for the wall of text. Feel free to correct me if I'm wrong or add anything I might have missed.
P.S. I personally think Scooter Imp is the best pet for Divine Link, that DEF/PROT aura debuff is too good and can hit orbs while mounted! Also I hear Kokopo is a pretty good pet.
EDIT: As an aside. The main detractor from Bone Dragon is it's inconvenience (Slow/Big). So if you don't mind those, it's a good candidate for a Lv200 pot.