Well, WordPress.com it’s the largest deployment of WordPress Multisite and they’re basically using the same setup (multiple tables for each site), so you could say that it’s a good strategy.
This kind of setup would let you to, for instance, keep your VIP users on a dedicated box, or add multiple DB servers as the network grows.
I’ve had experience with networks of about 500 sites and some basic tuning it’s enough to keep the sites working.
Also, check the HyperDB plugin, which it’s a replacement for WordPress’ default DB class that “supports replication, failover, load balancing, and partitioning”