There are many reasons, from philosophical to practical, why you would want to remove a citadel. The most pressing is if you are trying to secure space or an objective. Leaving enemy structures in space you just took give your enemy a place to stage out of. Being able to constantly respawn and reship in your system, or base out of, is major advantage for them. This applies for all areas of the game, from sov nullsec, to Faction Warfare space, to wormhole evictions, to even highsec if you want to try to claim an ice belt by force. Fighting over structures as part of winning that objective is one thing and that is good and maybe structures are reasonably balanced here with only a few tweaks needed (void bombs come to mind here), but removing undefended structures should not be pure tedium and require hours and hours of multiple people's lives to do if the owner has been expelled from that space or quit the game.
More philosophically, this game is suppose to be about conflict. It's great that people are building and deploying structures, but if no one is fighting over them because they are invulnerable 97% of the time and that 3% vulnerability is in timezone 10% of the players regularly play in there is an issue. Things in Eve are meant to be fought over and destroyed, not made so horrible, or even impossible to even try to contest with moving vulnerability window games that no one bothers. At the very least, abandoned structures should have a way to be cleared out at a fraction of the effort required to attack a maintained/defended one.
I don't care that they are piling up everywhere and acting as an eyesore, at least not yet. I am much more concerned about the ability to tedium-tank and time-zone tank with them and spam them everywhere to the point no one can or will spend the dozens of player hours to clear them out of newly conquered space. Until something is done about that, they will not fulfill their potential as content generators, and worse, asphyxiate the game and increase stagnation by dramatically increasing the number of hours required to take and secure space.
Edit: So a proposal to fix this? Add a token ISK or fuel cost to Upwell structures that requires a minute of player action every month to refill or pay. You can even waive that maintenance cost if a fuel-using service is fit and active. But if your station becomes 'abandoned' because no services are running and no maintenance is done, tethering is disabled (just like the Force Field on an abandoned POS went down) and the shield reinforcement timer goes away. Further, to reduce the number of player-hours to clear them out, entosis modules can now apply damage equal to the full damage cap on an abandoned structure (just like the original Upwell structure proposal). If at anytime before it explodes a defender shows up and "reactivates" the structure, tethering immediately turns back on and the entosis module stops functioning and the attackers again have to use a DPS fleet to attack it.
This not only makes abandoned structures much easier to kill while leaving defended ones as quite defensible, but adds back a minor amount of management that will put some tedium back to the defender's side disincentivizing spamming them everywhere as a strategy. It also is very similar to the POS paradigm we already have, and opens the possibility of someone forgetting to pay a bill (or pretending to do so) sparking conflict.