There is a drawback to doing that. The further in towards the carrier you cut and weld, the more the warpage will effect how far the axle bearing end of the tube moves. As far as using the axle as a jig, that's not going to do a very good job either. If you get a chance try sliding an axle in without the bearing on it and you'll see even when its slid into the splines it can be moved a lot. Whenever an axle get narrowed its always best to do it as close to the end as possible to limit warpage. A good weld in a V'd tube connection so you get good penetration is plenty strong so shearing is very unlikely. Using a pipe cutter puts somewhat of a V in the tubes but if a chop saw is used its best to chamfer the ends without going all the way to the inside. Without some kind of V and letting flat tube ends butt together then laying a weld on top of the joint is much weaker and should be avoided.