Guild Wars 2

Now that the Skill #3 on both Tomes affects the caster also these Tomes have become more viable in a few different ways.

However, casting Protective Spirit will give the Guardian +10s of Regen and Protection but when exiting the Tome the Protection is removed as it is part of the initial skill transformation, even if you gain additional Protection from another player while using the Tome.
This is very limiting and diminishes the different possibilities with this Tome.

Yeah, good catch on that.

I noticed this before when I saw that it gave protection, and figured maybe I could pop it and drop it for another way of keeping prot up, but to my dismay, the developers did not intend for it to be used that way so they made it drop boons on removal.

Side affect to that is exactly what you pointed out, additional boons added after the fact are wiped as well, even if they were not caused by the transformation.

Is it protection/regen only, or all boons?

Just Protection, so jumping in casting 3 and leaving will see you with 9s of Regeneration left and no Protection.

Tome of Wrath on the other hand you can jump in cast Might and Swiftness, pop back and still have both.