It's good because it keeps people playing from both ends of the spectrum. Many here have admitted that without the cap they would grind out their gear as quickly as possible, potentially then unsubbing once there was nothing left to do. This is probably the more significant reason.

For me (and probably other people like me) the cap motivates me to bother. I feel like I need to get at least 450 tomes per week (not currently though since my play time has been so little lately that I'm only 58 on my highest class currently). If it were uncapped I'd just say, eh, I'd rather not run this or that right now and I can always make up for it later, probably getting a measly 200 tomes weekly. I would probably work on just about anything else while thinking that next week I'll just run a ton of dungeons for tomes to make up for putting it off.

Probably seems odd, but I definitely accrue more tomes with a cap in place than without.