well speaking as an exclusive breastfeeder, an exclusive FFeeder and currently a mix-feeder I'd say a part of it is down to personal choice - ok my reasons for DS3 having a bottle of FF weren't entirely my choice, but by the age he is now (7 weeks) I had already planned before he was born that he would be getting a bottle of something at least 1 or 2 times a day (I was thinking perhaps Vodka - but DH says "hands off" his posh Polish Vodka, thought maybe Brandy instead ).
As it happens although I'm pretty succesfull with expressing I don't get vast amounts and certainly not enough on a regular basis to give him 2 (or more) bottles of EBM a day. So he mixes and matches.
My reasons for this.....well they're MY reasons and I don't have to tell you
OK - here's a few
So DH can feed him (especially at 2am )
So I can spend time with my older DS's (the first few weeks of exclusive BFing they really missed out - and I couldn't "afford" to let that continue for much longer)
So I can get back to playing the organ and church (yes completely selfish, but unlike my "day job" it's part of who I am, and DH assures me - from past experience - that extended periods of me not doing my music makes me irritable)
So I can start getting my life back into some (very) vague sort of order. Unfortunately as lovely as sitting BFing DS3 all day was it wasn't cooking the dinner, bathing the older DS's or putting them to bed.....
oooooo I can't be bothered - I think I'll just get my glass of Vodka and sit back and watch the show