Why should you buy a product that the first thing you need to do with is flash a 3rd party firmware onto it?

You shouldn't. But if you were to buy a router not realizing that it's crap, at least there exists a way to improve your experience without your having to drop money on yet another router.

Shouldn't the functionality and stability that you get with dd-wrt already be there in the original product?

Yes, but it rarely is.

I don't understand how Linksys seems to get a free pass on selling crippled products by letting the open source folks basically do all the work.

I don't give them a pass at all. I refuse to purchase Linksys routers these days.

My current router is a Netgear. Only after owning it for almost two years have I decided to flash it with dd-wrt, and only because there were two pieces of functionality that I wanted access to that weren't provided by the router:

1. The ability to forward an external port to a different internal port. I have no clue why this wasn't an option in the base firmware, but it wasn't.

2. The ability to monitor total network traffic. I'm growing more and more concerned that my ISP might eventually switched to metered service, so I want to know exactly how much data I'm transferring (and the only way to get reliable data is to monitor it on the interface between the LAN and WAN, and not just on one PC).