I don't think anybody is suggesting that Dallas Clark is the same player that he was in 2008-09; that is obviously not the case, and anybody suggesting that is delusional.
On the other hand some of the negative portrayals may be a bit over the top as well. Last year Clark did have 47 receptions and four touchdown receptions. While it is true that he just turned 34, it is also true that he is further removed from his wrist surgery.
Clark was never noted for his blocking, that is not and never has been his forte. The Patriots have plenty of other tight ends who are stronger in the blocking game than they are as a receiver. The Pats now have a hole at the position for a TE who has good pass catching skills. Because of that I think it would be worth at least considering Clark, or another veteran TE, and not so quickly write him off.
Signing him (or another vet) does not mean the team is depending on him this season; all it does is fill a spot on the 90-man roster and give the team an opportunity to better evaluate him during training camp.