Carl, I've not used a brand called flexbond, but do regularly use TEC's Full Flex thinset mortar, which would sound to be the equivalent.
The slight flexibility of these mortars doesn't mean no cement board; they work great with Durock, HardiBacker, etc. The advantage in the flexibility is that it will compensate some for expansion/contraction in the subfloor/underlayment surface (particularly wood materials) and minor cracking which may occur in slabs with tile applied.
Yes, it could mean that grout lines may crack, but better that than cracked tiles, right? You can also use grouts which contain some flexibility characteristics to decrease that possibility.
Overall, the true amount of flexibility in the mortar is minor & won't LEAD to cracked grout, but it will protect the tile from cracking or popping loose as easily when there is a risk of the base having some movement.