Being a mom, I can tell you that personally, I'd appreciate a handmade, washable, easy-care yarn for a blanket, and it wouldn't have to be a natural fiber, either. I began learning how to knit before I found out I was pregnant this last time, and once I found out, I was determined
to knit my baby a blanket and finish it (with the other two kids, I began crocheting them blankets but never progressed beyond scarf sizes!
) I saw SnB's "Big Bad Baby Blanket", but didn't want to spend out that much on yarn for the Koigu, so I got some Paton's Classic Merino Wool instead for the blanket. Although the blanket is very nice and warm and looks great, I can tell you that now that she's almost 2, that blanket has been hand washed more times than I can count, and waiting the few days for it to dry is frustrating. I just finished knitting her an acrylic blanket to use all the time and especially when her wool one is being washed and dried and she LOVES it (it's out of Lion Brand's Homespun).
In my own personal experience, I have found that for people that don't craft--knit, sew, quilt, crochet, etc.--it's not usually important to them that you used say for example 100% cashmere to make something...it's been my own experience that they're touched that you just spent the time to make them something. And sometimes, people are a little bit intimidated by things knitted out of finer things too...I know that my mom and sister don't want to ever wash shawls I made for them!
In my humble opinion, if it were me, I'd go ahead and use the Simply Soft yarn you already have. Even if you suspect they might be a bit put off by an "acrylic" yarn, I can guarantee
that after enough midnight/wee hours of the night trips to the washing machine after leaky diapers, etc., that blanket will be the most loved and useful out of anything else you could've knitted it out of, and they'll be THANKFUL you chose an easy-care yarn! Not to mention that Simply Soft is just that--very soft--and it has a nice sheen to it. And chances are, when the baby gets old enough to voice his/her opinion about it being their favorite little "lovey" blanket, it will become their
favorite too, because their child loves it so much (and because it's so soft!). And if you suspect they might be the least bit put off about something homemade, I'd be even more apt to go cheaper with the yarn because it really would be frustrating to me to go out and spend a fortune on yarn for something a friend of mine never used. Just my two cents' worth, though.
I'm very glad you posted this question, because I'm in the same boat. I have a friend that's pregnant, and she really likes the finer things...things I can't afford! So I've been wondering what I can make for their baby, and actually, now that I've reasoned it all out in my response to you, I think I too will select a more economical, manmade fiber. So thanks for posting!