We could debate this all day, because it's mostly subjective. I'm not trying to brush you off, but I can't think of a way to say it that isn't just repeating what I already said.
Although, I do have one thing to add: you make excellent points about the increased viability of stuff based on making sure that they always last 9 levels at least. But let's assume that you have the choice of upgrading a spell every 5 levels or every 10 levels, and it then lasts for 9-14 levels.
1. Those are identical early on in, and utterly pointless.
2. Or the one that is higher-level and will last for 14 levels is awesomely better and makes everything easier, thus making people complain about the uneven difficulty.
3. Or the one that is higher-level and will last for 14 levels is what you really need unless you are insanely good at the game, making the game way too hard if you don't grind the gems all the time.
The basic rule of thumb that I have learned from AI War is that if you make an un-fun or repetitive activity that can nevertheless be used to gain a gameplay advantage, players will undertake that activity repeatedly and then complain about it constantly. For many of them there is no "I'll do this when I feel like it, or have a special need." It's "I want to play optimally, and you've made the 'optimal' way un-fun, and I want you to make it fun."
We're also striving for a balance between simplicity and complexity here. Especially right when you are getting started with the game, having to suddenly upgrade your gems right as you are getting the hang of things doesn't add anything but extra complexity. Just as players are hitting level 5 and getting some new kinds of spells, and other parts of the game are unlocking, they also have to replace all their existing gems? Or at least have the option? It just doesn't work out very kindly except to the very hardcore, in my opinion.
In general I'm quite attached to the 10-levels thing instead of the 5-levels thing, for a whole slew of reasons including but not limited to the above, in short. It's one of those things that, to me at least, "just fits" -- like starting with 3 hearts in every Zelda game. There are a lot of parts of the game that aren't finished evolving yet, but the tiers-to-levels relationship I'm feeling pretty solid about. Never say never, of course, but you know.