I don't really know much about memory controllers, but being able to mask at the byte level seems like an important optimization. Without that, many writes will have to do a read first to them merge the read bytes with the dirty bytes.
The capacitors inside of DRAM cells are so small, that the very act of reading the DRAM cell obliterates the data. I'm not kidding.
The "Full procedure" of reading a DRAM cell is:
1. Row-Address -- Load a "row" (usually 1k to 8k. DDR4 is 8k IIRC) to the sense amplifiers. Sense-amplifiers can indefinitely hold data, but there's relatively few of them.
2. Column Address -- Once loaded, you talk to the sense-amplifiers.
3. Precharge -- You begin to move the data from the sense-amplifiers back to the DRAM cells. Again, step #1 obliterated the data, you have to write it back regardless.
4. Row-Address -- After the old data is loaded, you send it back.
So regardless, you have to Read-then-write EVERY time. In fact, DDR4 has faster write-speeds because you don't have to do the read step if you are only writing.
While this is true, the point was that it's an important optimization to avoid doing RMW at the memory controller level. If you did it there, it would cost tens of nanoseconds. Doing an on-die refresh in parallel with the write is almost free.