Like all things, you can do a cost/benefit analysis of the spire. The costs are pretty easy to measure provided you take the time. Bht benefits, on the other hand, not so easy. What is the benefit of having the "glorified wishing well" (I think somebody called it that) ? To some it might be the "bragging rights" of having completed it. What the value of that? Ranking points? coin and supply instants?, spell fragments? Even the value of the portal profits is relative and a product of the perception of the one valuing it.
What makes something valuable is, generally speaking, it's rarity and portability. If it's rare you get psychological value in having achieved or found it. And if it's portable as well, meaning you can sell it for something or a lot of other things, that makes it twice as valuable. How does the spire stack up to this?
The prizes are not rare. Available in almost any event and probably at a lower cost. Except for one or two I guess. Are the one or two (I'm not sure of how many unique to the spire prizes there are so I can't say but it appears to me they aren't exceptional in their impact on the game, beauty, ability to impress, or portability). Thus, the cost/benefit ratio of the spire may be less favorable than other events, tournaments, and fellowship adventures.
In addition, any cost benefit ratio would need to ask the question of the impact of this on that. As many have noted, there is the cost of draining resources from tournaments and even if you have 1000 instants to allow you to keep up with both, eventually you will have less than 1000 and then less than 900 and then less than 800 and so on until you have less than you need to do both. Of course, it might be fun in the mean time and the fun you get is part of the equation as well.
Now, given this, if the devs really want people to play the spire they need to make the top prizes so extravagant, and the effort to hard, that only the elite of the elite make it each week to the very top. I'm not sure what such an extravagant prize or set of prizes might be but they should be something you can only get from climbing the spire to the very top, spending insane amounts of resources and troops, and which has a lasting and huge impact on your city and status in the game. "Spire Hero" should be a rare badge of honor worth earning and displaying and mark the player with "rights and privileges" befitting such a hero.
In the end though, as it stands, for me, it's not worth much effort if any. And I'm going to encourage my fs to avoid it as we make an effort to move into the ranks of the 10 chests fellowship list.
AJ