The in yuniverse explanation about wasa reimus is probably as simple as they were born with a shorter hair tube which is why the ends are all fluffy like that. It is a defect for sure but I doubt it is that physically debilitating unlike a shelled marisa which is very likely to have a hell of a time moving around.
They probably COULD compact it to a regular reimu's braid shape but would not naturally want to bother doing so because it would demean their specialness (like spoiled and entitled humans would not want to do things they consider beneath them because they are so used to be pampered by others).
Anyways about the braidless yukkuris being normally depicted as not using their hair limbs, that is just because of two main reasons:
1) they look awkward for the most part as their logical hair limbs are not where you'd expect a human hand to be placed if you give human like anatomy to a yu which is the upper part of the head as the head, the hip area to the body and so on.
Marisas, Meiling, Sakuyas and those with prominent hair features at the side of the head but not too far away to the back are very easy to draw using their braids. Add the fact that they normally have ribbons that clearly define those features very prominently, you'd see how easy it is to draw them.
2) most of the artists that draw yus are doing it for the attention for the most vocal and probably numerous members of the fanbase that are of the "yukkuri-are-weak-and-stupid" variety so they tend to not be very creative and just go with the typical tongue limbs for most of the time with the exception that Marisas and Reimus appearing to be using their hair as limbs.
There were a few pics of the yus in the past using their hair as limbs but they did not catch on because it was probably thought that it would be more humorous that yus use their tongues as limbs. This also, probably, make yukkuris appear even dumber which seems to be what the majority wanted or was more comfortable with at that time (about the same time a rash of ankos and english fics started gimping yukkuri magic as harmless to humans -_- and even ayas were depicted as not even faster than a human child. *SMH*. Talk about insecurity issues).
Anyways, I actually created a topic regarding hair limbs sometime this January when I was secretly gathering ideas for my yukkuri sim rpg text game.
http://oyp.yunyah.com/forum_topics/10426 and for the most part back then I was not imaginative enough to picture out forelocks and sidelocks as hair limbs and it was only when I started getting sick of drawing Reimus and Marisas that I looked at the viability of sidelocks and forelocks. The process is pretty hit and miss because the hair limbs are either too long (Satono and Mai) or too short (Koishi and Myon) if you want to keep the resemblance between the touhou character's head and the yukkuri counterpart..
I rechecked and back then Eibura actually made most of the art depicting yukkuri other than the usual Marisa and Reimu using their forelocks, sidelocks, tassels and ears.