One of the posters on WotC
if wizards don't sell spells or scrolls it will cost more to get spells. have to trade a spell or magic item which means research costs or construction costs. means that spells would be much rarer and harder to get. also spells would be much more unique. wizards will cast spells.
under d20 SRD research of level up spells is "free" in regards to time and money. also no minimum time needed to do such research. the lack of research time is incongreus. if research for level up spells is free, all new spell research should be both short and cheap.
one arguement was that wizards learned new spells by improving existing knowledge, or knowledge learned as an apprentice. if based upon using existing spells, then level up spells should only be drawn from spells similar to ones already known.
A point was made that wizards hoard spells. While this is to a certain extent a GM perogative in world design, it helps to look at how others have done it.
sudden development - level up wiht new spells and skills with no down time or training requirements.