Well look at it this way. You realize your 1B is not in the lineup for that game, so you just go on the free agent list and grab a guy and start him... thats what your saying. Do you think you can do that in real baseball?
No but in real baseball you have someone on the bench that you can insert in that player's place whether or not they have yahoo 1B eligibility. I believe in the MLB you can option players up from AAA the same day you intend to start them also.
Also, in yahoo fantasy football, you can pick up a player and start him on the same day.
It's a long season though, so though I agree with the poster who said he wants to be able to add and start a player on the same day, I don't think it's a big deal.