In my opinion, if someone plans to use the XL as an all-in-one multi-effects in order to do everything, then it makes more sense to get it instead of an HX Stomp + a MIDI controller. The XL wouldn’t require any MIDI programming, MIDI cables and any dedicated power source. Also, it’s footswitches are touch sensitive and color coded, which really helps with the HX interface and looks really nice.
A MIDI Controller + HX Stomp combo CAN be much more powerfull than the XL, even if it's used as an all-in-one solution, but it would be much bigger and much more expensive, so it's really hard to advocate in favor of it.
But I disagree when people see the XL as a good adition to an existing MIDI capable pedalboard: as soon as you start to increase the number of pedals, or the complexity of them, the XL will inevitably fall short because the Command Center on the Helix is fairly limited. So, depending on the pedalboard, a combo consisting of a good MIDI controller and a regular HX Stomp would be vastly superior.
So, at the end of the day, I think the XL will appeal only for those planning to use it as an all-in-one soluttion (with somewhat limited DSP) or for small MIDI capable pedalboards.
My particular wish is that Line 6 make something like an HX Stomp XL, but with the DSP capabilities of a full Helix, so it would be even smaller than an LT and you wouldn't be stuck with an expression pedal if you don't use it frequently, but you could still make use of the poly effects or double amps chains in a reasonable manner.