I think it depends. For Xerath I can actually see why they gave it to him. It exposes him to some level of risk, yes, but look at the range of his spells: if he had a way to gain mana back that let him maintain that range then how do a lot of the mid laners attempt to deal with him besides overextending to Hell and back just to damage him? It forces some level of interaction, and while that's not great for the fantasy which he attempts to embody, it's good for forcing some level of interaction with the enemy besides endlessly poking them outside of their own sphere of influence. His functions as much for him as it does for the enemy and gives them some sort of window to act against him without having to jump into his zone. Do I think it justifies the cost of the spells themselves? Maybe not, but if he didn't have something to force him closer then he'd simply never let the enemy interact with them and that's terrible from the enemy's perspective. It's worth remembering that his fantasy theme feels really good to play as when it's working well, but it tends to feel very terrible to play against if he never has to expose himself to real risk other than the longest of dives or really wide flanks.
Lissandra would be a much better argument for a champion whose passive doesn't justify the cost of their spells. She has a free spell every so often, but due to that her spell costs are generally pretty freaking high across the board because if they weren't then you wouldn't feel good about using your free spell on it, and if her spell costs were low then that defeats the point of the passive at all then, doesn't it? Her passive actively works against her own balance as a result, because that's all it does. It doesn't force interaction with the enemy, because her sphere of influence is small enough that she has to interact with them anyway, and it doesn't help her mana issues because its design actively encourages those mana costs in the first place.
There has to be a risk vs. reward balance kept for a champion's design, and if you let Xerath stay in his comfort zone then he's pretty safe given the elements of his kit. He has pretty long range, good damage, a hard CC, but the trade off is that to sustain all of this he has to expose himself to the enemy until later in the game when items can provide that sustain for him. Lissandra already has the risk element which is why she has more CC and direct survivability, but lower damage and range as her trade-off. Her reward for that? A passive that actively works against her own balance.