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A PR0, as opposed to a 2 or 3, may be a little different but still it doesn't tell you a lot. On the other hand, it doesn't take much effort to achieve a PR4... I guess the point is that if you're shopping around and find a firm with low PR on their own site, unless you're going to ask them to explain it to you the conclusions you jump to may not be accurate.
You may have read, too, some threads here where the point was made that a lot of seo firms want to stay below the radar and so make no effort to get high rankings on their own site. An extension of that would logically be to make no effort to increase PR.
And check out the links on high-PR seo sites... one technique used is to place hidden links on every client site back "home" -- links that the client may not even know about. So does high PageRank gained that way mean a good seo firm? A matter of opinion, I suppose.
Maybe it is being harsh, but there has to be a balance between optimization and conservativeness on the part of the client. I doubt 95% of the "firms" out there can do this - or let their clients know. A PR0 means one of two things - too many tricks - or not enough optimization.
A zero probably means Bad Things were done, while a three might mean a number of different things: new site or domain name, no effort to build links, putting all the time and energy into clients and not their own site, relying on other marketing techniques instead, trying to stay under the radar, penalty, incompetence...
A PR3 might be a warning flag, but I wouldn't look at it as something that would mean automatic disqualification.