I don't know how the minds of all researchers work, but creating definitions with the sole purpose of recieving credit for them is pointless. Definitions are crucial to research, not because you can get recognition, but because definitions are the prerequisite for talking about a concept. If I can't define what a blog is, I can't discuss it, because I can't distinguish it from other forms of communication. Following that, the idea of keeping a definition as open as possible is a bad idea. While a very open definition where a blog or a videoblog can be almost anything might be 100% correct, it'll also be 100% useless.
First I want to address this comment.
Some wide definitions work quite nicely, and lets us discuss objects of research as more than a narrow category. Take the semiotic understanding of "text". It permits just about everything which is created with the purpose of sending some kind of signal to be interpreted as a text. This positions the object of perception as a cultural artifact, and lets it be interpreted according to the context in which it is perceived. It is an extremely wide definition, as it encompasses almost everything made by humans, and according to the logic of Pedersen it should be almost useless. But it isn't, quite contrary.
Sometimes, in order to discuss a phenomenon, we need to delay the process of narrowing it down. If I am to find out what a blog IS, I can't just look at the blogs that adhere to journalistic standards, academic blogs, disaster blogs, travel blogs, personal diary blogs, photo blogs, moblogs, voice blogs, video blogs or what ever. I can't understand the BLOG from either of these narrow categories. I can however learn something about a certain type of blogs, if I make it clear that my study is only about this.
But what made me react originally was not really the idea of a definition, although definitions and manifestos are related.
If there is a bloggers manifesto, declaring what a blog should be, and this is so narrow that any of the above - and several unmentioned - types of blog does not fit, then the manifesto is reducive, and changes a phenomenon that already exists within its own boundaries. It's not a definition (which a manifesto is not, anyway), but a statement of the writer's intent for what blogs ought to be. A definition is descriptive: "This is what I understand this object to be." A manifesto is normative: "This is what this object should be."
When Adrian Miles wrote the vog manifesto, the vogma, he was quite alone with the term. As others found it and realised "hey, this is what I am doing", the term became established and took hold, and the vog developed from Adrian's intent. The blog has some technical features and some intent such as "push-button publishing for the people" - a slogan, but also a type of manifesto. But there were no original rules limiting a blog in for instance genre, content, ethics or graphic expression in that early phase, and the blog developed until it had reached millions of users (and budded into for instance vogs) before the desire for a manifesto or even a definition arose.
The vog was created through the vogma, the blog risks to be reduced by a "blogma". These are two very different situations.
As stated before, I am not all that concerned by such a reduction (if it does take place, that is not a given), as I expect the practice will just find other channels - or not change at all. A reduction will however be a loss to research, as it means a lot of ways to use a blog can be lost to the scholarly eye. I am old enough to remember when comics and paperbacks were not literature, and to know what a struggle it was to put popular media into the Universities. I don't want to participate in creating an academic exclusiveness that excludes the weight-bloggers for instance. Definitions need to exist at several levels, as they have to describe what already exists. Manifestos can be narrow and restrictive, because they describe what we want to see come into being.
But who knows what will come of a bloggers manifesto anyway? Some brilliant mind could do both: contain everything that blogs are at the moment, and point towards some bright and beautiful future. I would love that!