10 books does seem just a bit excessive...but only because of the extra fees you'll likely incur if you're playing at places that don't give you free withdrawals. But aside from that, why not? You need a place with good dog lines, another good for favs...another one (at least, maybe two) for reduced juice...scoop a couple good bonuses in from somewhere else, etc.
If you've got $10,000, why put $5,000 each in two accounts when you could put $1000 in ten? If I did that I am sure 3-4 of those accounts would be wiped out by end of season.... but a couple more would be about level when it's all done, and the other 4 would show profit. In the meantime, the more outs the better.
And those extra half points are definitely worth it, for exactly the reason BBC stated. Every extra % point you can add is a lot in the end - as long as it more than offsets those fees.
(edit) I know I learned one thing from last year. I am reading bonus policies a lot closer this year. I refuse to play anymore at places like Carib that are going with this new trend of giving you "more betting power"

in the form of bonuses that can never be cashed out. The only way you ever access the bonus is to put your entire roll at the place at risk at one time. So they just want suckers signing up with them. Pass.