Ties last longer with heavier rail. Light rail can split ties down the center or rail can cut into tie --- even with oversized tie plates. Also, ballast lasts longer with heavier rail, and ties don't grind in coarse ballast with heavier rail. 6 bolt angle bars were available for all rail sizes, but not all roads wanted to the expense. Welded rail prevents shelling at joints which can lead to compound battering or corrugated rail. Welded rail also prevents the problem of bolt hole breaks and split heads. Welded rail also reduces the need for tamping as joints tend to sink due to the slight hopping that takes place at even the best joints. There are a handful of minor problems that welded rail reduces or eliminates. (There is also the issue of joint bar designs, for example, Union Pacific's wrap around bars used on lighter rails were much better that designs used by Northern Pacific or the Great Northern on rail up to 115 lb. NP & GN later went to a more standard design on rails heavier than 100 lb.) Properly installed welded rail is far better than jointed rail as far as I'm concerned.