They're not against third party grips either usually, but because its a Nintendo made thing that adds buttons and a stick, I think people just vent their frustration that the stuff wasn't built in without really asking themselves if it was really necessary to be built in and whether comfort is perhaps more important. You can count the games that use this thing on one hand.
I don't necessairily disagree, but I think a case can be made for building in a 2nd analog/L2/R2 on the XL without adding much/any bulk (was considered on 3DS originally but decided against due to lack of space in the desired formfactor) and if Nintendo had done so I'd think the selection of games using then would've increased too. There's games like EX Troopers or Luigi's Mansion 2 that would definitely benefit from it, but aren't using CPP now. It also could've encoraged other games coming to the system too, the complete lack if fps on 3DS for example might've turned out differently.
edit: Spoke too soon, it seems EXT does indeed use the CPP! Full list of supported games:
Ace Combat: Assault Horizon Legacy (Namco Bandai) JP version only
Dynasty Warriors VS (Tecmo Koei)
E.X.Troopers (Capcom)
Kid Icarus: Uprising (Nintendo)
Kingdom Hearts 3D [Dream Drop Distance] (Square Enix)
Metal Gear Solid: Snake Eater 3D (Konami)
Monster Hunter 3 Ultimate (Capcom)
Monster Hunter 4 (Capcom)
Nano Assault EX (Shin'en Multimedia)
Resident Evil: Revelations (Capcom)
Samurai Warriors: Chronicles 2nd (Tecmo Koei)