You can indeed only use Dragon's Lair at the beginning of your turn, but with Purged by Flame, at least you can trigger it on your next turn after playing it.
Unfortunately Dragon's Lair is at best a second turn card. So you're looking at third or fourth turn to really get any use out of it. Thats already well on your way to losing IMO.
As much as I dislike arguing based on best or worst case scenarios: At best it's a first turn card (Convocation of Eagles, Innovation), and having a dragon hit the table in your second turn isn't half bad. Neither is getting one on your third or fourth turn, actually. It's not like you're not doing anything else while you wait for the Lair to do its thing. I'm not saying there aren't stronger approaches, and as always a lot depends on your opponent and your draw, but it's viable.
Winvasion - The Warhammer: Invasion podcast
The problem isn't Dragon's Lair but what to do with those high cost dragons before 5 turns (or getting the lair) !
I tried a rush deck based on dragons and Inflame/Fury of Aenarion (for at least 4P) but the results were very various.
I really don't think we are in disagreement. I've just had a few people look at these cards and exclaim "HIGH ELVES HAVE ARRIVED!" The Dragon's Lair deck is pretty lower tier even for HE decks in my experience. These new cards will probably be better for an Eltharion Deck or in something like my unitless HE indirect damage deck.
Just for fun: To get the Dragon's Lair to try and spit out a dragon on the second turn, you'd essentially have to have 6 specific cards in your opening hand and play all 7 cards you start with. Play Envoy From Avelorn for one, play a development (any card), play 2 Dance to Loec(can also be replaced with Convocation of Eagles), play Innovation to gain resources, Play Dragon's Lair for four, then play Elven Scout/Purged by Flame for one(and to put the resource on the Lair). Thats 6 resources and six or seven cards from your hand depending on your use of Eagles or Innovation. If you use Convocation of Eagles, you hurt your chances of drawing a dragon because you check the top five cards with Lair before you draw in the Quest phase. So possible, but pretty rare. And lord help you if the other side has any support removal In my experience, playing Dragon's Lair even in the second turn is still a chore. To pull it off you are essentially not doing much of anything else even then since it costs four (five to get any tokens on it).
Now, none of this is saying that I'm not going to make a deck and REALLY enjoy it if it ever works