Yeh I did consider this once before but decided against it.
I think bosses should remain semi-difficult to farm due to their location, spawn times and possible wars with other parties/guilds to obtain them. Having this item would probably destroy that entirely, especially if the item was available to purchase in some way. A cooldown would help I guess, but there would be nothing stopping somebody from buying 10 of them and handing them to their friends/guildies to circumvent that cooldown. So if it were to be purchasable it would probably need to be untradable. But then the person could just hand the moolah to friends to each buy one, so wouldn't work really.

Another thought I had back when I was deciding if I would add this is to make the item craftable or obtained by exchanging a certain amount of other items but I didn't want to restrict it to a crafting skill and we didn't have an exchange system back then, but when the pet update goes live we will have the new vendor exchange system, so it could possibly be used for this. That way, it would take a number of items to exchange for the summoning item, preventing, or at least making it more difficult, to get a lot of them at one time.

So, if this were to happen, it would definitely not be a tradable item, it would have a fairly long cooldown on it, and it would be difficult or time consuming to obtain. I very much doubt I would add Catacombs bosses to it either.

It could possibly cause problems like players summoning bosses in low level areas, but monsters spawned from scrolls disappear fairly quickly if they are not engaged. I can also see players spawning stuff in Fire Pit and Battlefields for the lols, but that would be a huge waste, but I'm sure some would still do it. I could probably restrict summons to normal field maps or pvp maps or something tho I guess.

I'm not sure about it really. As I say above, if it were to happen, it would have to be difficult to obtain - but then how will you feel doing the work to get one and then summoning a boss you don't even want.

moaaarr bossseeesss

I kind of think that would be fun to have an item like it. But at the same time it might be broken if it was sold for moolah just cause players could buy a ton and grind bosses or for some players summon all the bosses in an area or something like that. (Added like a high cool down might help)