Depends on the blog authority. If you have a high impact blog like engadget or problogger then even though it wouldn't be much link juice you are getting, commenting on them will still be valuable and you might gt good traffic as well. You must also see if the blogs are do follow. By default blogs will have comments no followed. But many bloggers make their blogs do follow in order to attract more comments. Also make sure you do not use keywords to link back to your site. Webmasters usually dislike this attitude. You can link to your site at the end of your comment. Just make sure you have something worthwhile adding to the comments
I've checked a number of websites that are placed number 1 in Google for keywords that get huge search volume and the sites are competing against the likes of another 30 million + other sites for the same keywords. Even for these sites that are at the very top of the tree, they do not tend to have many links back that give really good link juice, even though some of the links are from websites with PR's of 7,8 or even 9. The point being, its unclear to me what really makes a fantastic link back to your site according to Google. Some people say its not so much the PR but rather the amount of pages the site that is linking back to yours has indexed along with the fact that its often cached (updated and visited by the spiders). Agreed, you can just go for getting links from high Alexa ranking sites such as ezinearticles.com, Digg.com, StumbleUpon etc and it does not matter about your anchor text in the link (which can be very difficult to attain anyhow as has been mentioned by eBusinessCorporate) at all because the main thing is to get some of the traffic from these hugely busy sites. Mind you, this is easy to do in practice - get the link I mean - but to get the knock on traffic...???
Philosophically spoken - it's a valid question, but why worry? There's nothing you can do about it anyway. I suppose you'll have to keep creating new links anyhow, so you'll be OK.
I would agree SEO Michael but these obviously have to be DO FOLLOW blogs for Google to pay attention (MSN and Yahoo do not seem to mind either No Follow or Do Follow) which it would seem are becoming less and less - or is that just me thinking this?? Furthermore, the best quality links are those that are anchor text with your choice of keywords that are WITHIN relevant content to your keywords - or the so-called LSI effect. This is what Google is looking for anyhow, although I don't believe Yahoo or MSN worry about it for their ranking structure.
Add some good external related links so that the more external links the more chance the site will be index.. add ur site here: http://www.google.com/addurl/