Since P3D v4 was released I have been looking to see if i can use old airport sceneries that I have purchased in the past. Thanks to the list that was posted to show what sceneries were compatible with P3D v4, I have manged to install lots of add on airports and they all work fine. But more and more of those exact add ons are being released for P3D v4 as new versions, some charge for the update and some don't. Now the question that I have is what do we get with the new versions of these sceneries? the ones I have work so why would I want to buy the same airports over again? I don't really care for new lighting or better grass and things like that so when that is the only things updated does it make sense to buy them again. I am very happy that these sceneries are being made for P3D v4 but I am confused on what makes it so much different from the old version.

I really think you answered your own question, the logical upgrades would be to use the new v4 effects....dynamic lighting and other things along that line. If they currently work and you don't care about the dynamic lighting etc, then I would agree that there would be no point to paying an upgrade fee. I would also think what's in any particular upgrade would best be answered in the publishers upgrade summary (sometimes here-say is wrong and faulty).

I agree that it would really depend on what is updated. If it's just grass and various fixes then it's not worth it unless my version has problems. If there is a new terminal or runway then it might sway me to buy it again. I am grateful for the small update pricing but those can add up pretty quickly if you know what I mean.

They are also taking advantage of the performance and not having to deal with VAS.  Devs can now put in all the little things they left out for v3 and 32bit platforms.  Animations, lighting, grass, extended size of airport, textures, etc.  They put in the time and effort to update.  You pay $5.  Seems like a fair deal to me.  

No one says you have to purchase.  Its a free country...well it used to be.  

Everyone has pretty much answered above, but one more thing you are paying for is support. Developers won't want to support an old product forever.