Why is it bad if the level of Archmage or Bishop is high when using leeches? The higher the level, the stronger leecher is, so isn't it good?
Regarding your concern, it's more about the amount of EXP the leecher is getting For example, leeching EXP from a level 120 at Bubblings would be better than a level 180 So, low level leech are usually done by 120+ mages
I think it's bad just for low lvl leech. As a seller and buyer myself i find the amount of exp on higher leech spots between high-low level lecher isn't that much to be a real concern but you know, people will fight for that small difference (which could pair with 5 extra minutes compensation from a high level). Anyways, that's me and a subjective thought, i don't mind the lecheer's level when i buy leech unless it's low level spots (mb1/mb2/wr and up to harps)
It actually is quite a meaningful difference, at least depending on what you consider as meaningful! Skele leech as an example can net ~37-38M exp with two 120 leechers (more if the leechers are both 13x) under perfect rotation if you're around 14x. By 18x this drops dramatically to ~32-33M per leecher, almost a 15% difference.
15% difference is not such a big deal, talking about mesos it's like what 5-7m? 5m exp as well. And about time I don't think it's more than a 10 minutes difference which isn't a lot, same time you will spend looking for a low level to leech you and for the mesos would be the same you're spending on smegas looking for a seller. I've seen people refusing to buy leech from 17x bishops wasting way more time and smegas to find a low level than the actual difference between them. You could just ask the leecher to give you extra 5 minutes for the level difference and I'm pretty sure most will do if you kindly ask for it.