I disagree. Harewood and Heskey had nothing going for them, and never did (apart from 2 or 3 'important' goals in their years here).
The players that did have potential or promise were either given ridiculous contracts or stupid transfer fees: Davies £10m, NRC £8.5m, Cuellar £7.5m etc etc
Regardng your point about other teams paying for players (Park/ Mertesacker/ Gervinho etc etc), I would actually be interested to see the difference between the bigger clubs' signings and their transfer fees, compared to ours over the years.
Obviously the bigger clubs have much more 'pull' so they can offer better quality players lower wages than we would have to, because they have the lure of European football.
But, inspite of that, I've seen players like Vermaelen (sp?) and Kompany go for similar prices to Cuellar and NRC.
Like you say, no managers get it right every time, but I feel that MONs success rate was rather low, when you consider transfer fee, wages & contract offered, sell on value and potential to improve or decline.