For one reason or another (mainly from reading lots of Jack Vance and Terry Pratchett) I've always thought of wizards in my campaigns as being generally mutually suspicious, competitive, and often positively hostile to each other. That's fine as far as campaign background goes, but it's not really usually very workable as an intra-party vibe if players want their characters to survive for long. Cooperation is almost always a better survival strategy than competition.
Players will often want to combine their magical resources, so that there's a degree of redundancy in their spell-books. That's a good idea from a practical point of view, but it doesn't fit well with the campaign background.
It's a simple matter, however, to discourage on-the-fly spell trading without arbitrarily forbidding it. All you have to do is make it a long, difficult, expensive and risky process, and it will happen rarely, if at all.
In my own campaign, I see magic-users' spellbooks as being like a cross between the Lindisfarne gospels and the Voynich manuscript; ornately illuminated and written in a unique personal code.
It's possible to copy a spell from one wizard's book into another, but it will require a whole lot of Read Magic spells, access to expensive inks and what-not, and plenty of time — at least 1d4 days per spell level. Also, there's a small chance (actual % depending on the ratio of caster and spell level) that the process will destroy the original, like reading a spell from a scroll. Even that tiny risk, along with having to surrender control of their spellbook for so long, makes most players unwilling to freely share their spells with others. However, it does make it possible for MU characters to expand their repertoire from found/stolen material as long as they're prepared to devote the time and resources to doing so.