You can try out all the professions with 5 character slots. It's another matter that you need to delete maximum 3 of them to achieve that.
Also, I can't see why you want to pay monthly for years, if said slots would cost only like 1 max 2 month's of sub cost as Sapho already mentioned.
Seriously, if A-net offers a hand, there are always people out there who want to get the whole arm...
It's not necessarily that I want multiple character slots. They keep telling us how other companies lie to us when we pay subscriptions, and that server maintenance and all the other costs are far less than said companies claim them to be. However, they force us into kinds of microtransactions that those other companies don't do, because you already pay monthly. If they just had said: "Look, we're going to have extra character slots in the store, DLC packs and transmutation stones" - and whatever else they mean to sell -"because that's how we plan on making profit", it would've been cool. Instead they're telling us that the store is optional, while denying us core game features without using it.
But nevermind character slots and other cosmetics. The fact that the game has no subscription could turn the store into an evil thing. And before you go and tell me that they themselves have said they won't let it get evil, think about the Mercenary Hero Slots in Guild Wars 1. Hell, it might be a 6 year-old game, and the only future it has is being milked for money, but how do they expect people to believe that they won't offer in-game advantages for money in Guild Wars 2, when they already have Guild Wars.
PS: I'm not a doomsayer, and I'm sorry if I sounded like one. It's just that I was thinking that as the game nears completion it'll get me hyped even more, but the more I hear, the more disappointed I get - this being one disappointment, the other being some of the changes for gamescom.