Let's talk about Lugia and powering one up.

As I mentioned in the other thread, Lugia can be a good generalist attacker if you are looking to take down gyms quick. It would basically play the role of Snorlax. HP would actually be the preferred move because then it would be perfect to deal with those pesky Rhydons that seem to be the most annoying for my Dragonite/Snorlax lineup.

Dragon tail as a quick move would also be a solid option against Dragonites.

Lugia served no purpose as a raider, other than being a 6th pokemon to hold down the fort at the end.

Lugia certainly isn't needed, but I would roll with a Dragonite/Lugia team for gyms.

The biggest thing for me is how quickly will the next set of Legendaries come out? I want rare candies and stardust for them, but I can get more stardust and I already have 300 candies sitting around.

Lugia can't learn Wing Attack.

I had my friend who just hit lv30 yesterday spending his last dust and rare candies powering up his Lugia as much as he could. He is very proud of it, especially since he is the only one who caught it among our 20 raiders group.

I wanted to tell him to set back and reconsider, but he was so proud and happy that I couldn't.
I'm now glad that I chose to mind my own business, as a lv30 rural trainer he doesn't have much firepower to get through gym and bosses, so Lugia is a good generalist for him. Nice catch buddy! :)

TL;DR: pretty much what you said-rs biggest pro is its bulk and its biggest con is its comparatively low attack stat.

As blue mentioned, you put WA when you meant to put SA (sky attack)

You pretty much summed it up; it's super bulky but the fire power is just lacking. For that reason, It is outclassed in raids by other greater-dps options.

In the main series it was even better because it had an ability that if it had full health, the attack it receives does 50% of the original damage. This is problematic when Lugia has this ability "multiscale" coupled with a healing move such as roost, that would bring it back to full hp typically, thereby repeating the cycle.

In Go it gets both psychic STAB moves and a flying charge STAB, but it is just outclassed by other psychic types with higher attack stats, which are necessary for raids with less people

I wouldn't spend any rare candy except on the following:
1. Mewtwo
2. Moltres
3. Raikou/Zapdos (Raikou preferred)

Other legendaries are all trophies to me. It's good to have a high IV one but it doesn't matter to me since I won't invest in them anyway.
To me, the point of powering it up, as of now, is only for attacking (previously for CP and gym placement but not anymore). So I will only invest in those who can fight the battles for me, who I can reliably count on. Since the places of my squad are pretty limited, only a certain species of legendaries can serve me better. For example, none of the birds/dogs/cats can do Machamp's job better.
As for Lugia. Can it sweep a gym better than Dragonite? Maybe, but it's too slow. I treasure time as I live in an intense and highly competitive urban area and time means everything. So he's definitely cool looking, and a pretty strong psychic type, but I'm sorry Lugia, you don't have my love.

Until either:
1. we will be able to place them in gyms and the gym defence will actually mater,
2. the raid time limits will be increased for solo players, in that case Lugia will be there like Snorlax and Blissey were before for soloing lvl 10 gyms

for me both Articuno and Lugia are just trophies. Same for every legendary except Mewtwo, Moltres, maybe Entei (I'm Valor :) ), maybe Zapdos.

I spend my rare candies on my 100 IV B/C ttar - and mewtwo when available - not planning on using dust on other legendaries.