1. Firstly (and this is probably the most nitpicky argument) If you go all out and take some version of the Nazrite vow, you would have to break it when you went on Hajj.
2. Even without dreads, when I would go out at night, random people would occasionally ask me for weed. (although I should probably add that I would sometimes wear a big poofy red, black, yellow and green "rasta" hat). In any case, I thought that if I went further and actually had dreads, the requests probably would have gotten ridiculous.
3. In Islam, there is a basic principle of not imitating the practices of non-Muslims and dreadlocks are pretty distinvely associated with Rastafarianism.
4. Dreads make it harder to do the ablutions for prayer (salat).
5. Dreads make it harder to wear a normal-sized kufi.
On the other hand, (to address 3) there are some indications that the prophet (saaws) may have had his hair in some sort of braid. And memebrs of the Baye Fall Sufi order are known to wear dreads. While the other considerations don't necessarily mean one shouldn't grow dreads, just that they come with certain burdens which one must be willing to accept if you want to grow them. (e.g. grow dreads but trim them after hajj, get a bigger kufi, take the extra effort to wash them, etc.)
I you really want to reflect more on the subject, I would suggest that you check out the above blog.