I haven't had too much of a problem with Spore / Burning Sun decks unless they were being played by Iczers who were significantly better than me (you know who you are). It burns a ton of SP, usually more than I will keeping my field alive. Let's say I have 3 units out when the combo happens the first time. I'll end up spending 3 SP to revive them all. My enemy will burn 5 SP playing the sun, plus another to revive the spore to do it again, so twice what I spend. Often, this will be done when the enemy has little to nothing on the field. I only actually have to keep alive enough to hold out against my enemy's field, which is generally one or two high level units, or often, nothing. Anything else I can let go to the cemetery, giving his High level unit some soul skill power to deal with, and use the SP I get from that to bring out something higher level.
So you ask, what if I don't have anything higher level? Well, then you're taking a risk playing that deck. Don't run anything high level? You can get screwed by Burning Sun. Don't run anything low level? You can be screwed by Succubus or Shade or Lawtia in general. It's actually a situation that rewards having a diverse deck. (Imagine that!) If there's anything you can tell just by reading the text on the Return card it's that this game is very level aware, and it really wants you to have a range to choose from in your Card File. This is something that was mostly (except for Return, again) missing from Set 1, and that's the value in having a high level. It's what sets it apart from casting cost in Magic, where a high casting cost was a pure liability. Here, there are advantages to having a high level card, in it's ability to "store" SP and keep it from evil Shades until you need it to bring out the right card for the late game, and it's immunity to certain game effects.
It looks like the Folrart Iczers are realizing this as well. There were lots of complaints with Set 1 Refess (keep in mind that we're actually getting posts here saying a Refess card is too powerful now) but the one that everyone kept coming back to was that "this card is too high level for what it does". Refess cards are actually very often just a little underpowered for their level. The idea is that this is a disadvantage, but having a diverse range of cards available to you at higher levels is also an advantage, because sometimes being higher level IS the advantage you want. If all the cards with the abilities you want to include in your strategy are level 2, the Witch is a lot more scary. Only now is some of that idea actually reaching the metagame with Set two strategies.
"Scissors are overpowered. Rock is fine." -Paper