Eh, I wouldn't be so quick to dismiss his complaints. It does add more complexity to an encounter (as in it's more to keep track of). My personal opinion on the matter is that this will (just like AoO) create a stronger need for some type of battlemat and minis. Whether this is a good idea or not...it sounds fun, but like the major complaint about 3.5, for the players. For the DM it's another thing to track (placement, movement and effect).
They've done this type of thing in D&D 3.5, I specifically remember the second room in the armory in Shatered Gates of Slaughtergarde where the wooden pillars holding the ceiling up can be destroyed to cause a cave in. I'm not sure how in depth or how integral I would want something like this to be for encounters. I like Exalted's method of basically leaving it up to the players and GM to use the scenery in stunts and providing a general bonus for integrating the terrain into their actions. It's when you start quantifying every actual specific effect (and thus creating more that has to be accounted for, tracked, etc.) that it, IMHO, it starts to become something of a headache. I mean let's look at this for a minute...
I have to keep track of...
1.) whether any PC/NPC or Monster enters a doomspore's square.
2.) whether any PC/NPC or Monster touches the doomspore.
3.) The concealment it's spores create.
4.) Which characters are bloodied and in it's area already, move into the area during combat or begin their turn in it's area.
5.) Make a Fort attack against said character for 1d10 poson damage.
6.) If #5 is succesful apply a weakened condition to target
7.) Again if #5 is succesful apply poison damage each round (that character doesn't save...I think) for the entire encounter.
In the end it just seems like alot to go through for a sideline enhancemment of an encounter. I think this may be one of those things that sounds good in theory and on paper, but has the potential to really slow combat down.