Recently, many blogs I read have been making some comments about Warhammer Online subscription rates.
And, I have to ask why this matters. EVE Online, which I consider to be a great MMO with a vibrant and active community doesn't even have 250,000 subscribers according to MMOGCHART. Those charts also say Planetside never had more than 60,000 subscribers, and I sure had a lot of fun playing Planetside for a couple years. If you subscribe to the monkeysphere theory, anything over maybe 500 people is just not going to matter to you as an MMO player.
There are, really, only two ways the numbers can influence the game. One is psychological, and the other is an indirect quality of the game. People might abandon the game under the concept that the game is not reaching it's intended 500,000 subscribers. Or that the game isn't as succesful as World of Warcraft. The more realistic impact is that the game is that the drop in subscribers based on estimates means a cut in services and support from Mythic.
In the end, though, I don't think that the difference between 300,000 and 500,000 will matter when it comes to the game. If it's fun, play it. If it's not, don't.