A good dryset mortar can be made into a modified mortar if you mix it with an additive rather than water. You can do it either way. The additive makes it stickier and stronger that may not be necessary or even desirable...just depends on what you are trying to accomplish. For example, all CBU (cement backer unit or cement board) call for them to be bedded in thinset. The thinset actually is there to fill in voids to give 100% support - the screws or nails are what hold it in place; so, there's no need or even desire for a modified thinset here.
Where the thinset can't dry ( installed on a waterproof membrane and a large, impervious porcelain tile), you'd probably not want to use a modified thinset. If you sandwich a modified thinset between two waterproof layers, it can take literally months for it to dry out and achieve max strength. On the other hand, a dryset mortar cures anaerobically - needs no air, so it would achieve design strength regardless of the conditions.
Regardless, you get what you pay for within the same type - a cheap dryset will have a bigger sand to cement ratio, and may only meet the minimum specs for strength. A premium dryset can have more strength than a cheap modified thinset.
Some people prefer to buy a good dryset, use it with water when needed, and essentially have a modified thinset if they use the addative...it lets them only have to buy one type of dry stuff.