My backstay is hydraulic. On this boat the backstay adjustment is mostly for headstay tension. Pumping it on does bend the mast, of course, but that bend is usually mostly removed with the runners to give the main some shape. But whatever it takes to get the job done...that's my principal rule. I race and cruise. Cruising is not lazy for me. Always optimizing performance and comfort.
I have the mast setup so that with the backstay off, baby off, runners off, not sailing, there is about 2 inches of prebend in the 60 feet of mast above the deck. The aluminum section is 8" deep. Then while sailing I adjust things as appropriate. 2" of bend gives a nice full main luff. No bend makes the luff a little too loose. More bend, maybe 8", a full diameter, flattens the main nicely, like a door, more so at the top. The conservative limit is 1.5 diameters, 12", of bend. I have the hydraulic cylinder set so that with no runners on I cannot go much past that amount of bend.
I went up in the bosun chair and measured the static bend at three places. The sailmaker uses those measurements for the sail. It is a truly beautiful thing when done right.
The babystay is really just there to hold on to while on the foredeck
It doesn't seem to do anything for bending. My spar builder agrees. It is rather an old fashioned thing, however is handy when bashing thru seas. Especially with bare poles or no wind.