I've been a PVP player for nearly 5.5 years. Without trained books, my alliance and I would have never been able to loan out as many guns to new recruits. With trained books we can skip a few levels and save half the fuel cost trading guns. Without trained, it won't be tempting for old players to loan guns to new players since we'll have to send 10 new guns every few levels.
Trained books save a lot of time, it lets PVP players use guns that are higher level, which is amazing considering not every Lr are fun to use. I personally despise SS but I love FAL's. Without trained, I'd have to go through 4 levels of boredom. I also don't like SVD's and M24's. Skipping 4 levels on SS, 4 levels on SVD means roughly 4 weeks of fun raiding instead of 4 weeks of boredom thanks to trained books.
There are plenty of players who don't like certain guns because of certain stats. SS lacking aps, SVD lacking suppression etc.
Another positive thing about trained,
When most players reach level 50 and unlock island, plenty of us have 12-15 level gap. Using trained lets us do island with half decent guns and crafted armors.
New players reaching level 50 don't want to start island with hp vests and level 30's AR's.
If you get rid of trained books, it pretty much doubles the trade cost when we give guns to our new players. Sure fuel is easy to come by for a max level player, but being a lvl 10-20 player and having to send back 10 guns every week because you out leveled them is quite expensive on fuel. New players feel like they are spending all their fuel trading and it's not fun since they can't save up.
Other players will have plenty of arguments to back me up and some will have arguments to shut me down. Trained books have been here for years, as long as they work correctly, they are enjoyable, not anti-fun.