Modern sealed cartridge bottom brackets are inexpensive, and do not require periodic service (as conventional unsealed bottom brackets do).

But there are drawbacks that make it worth considering to retain conventional bottom brackets that are of good quality, and still in good condition. These are the major reasons that I retain conventional bottom brackets that are still in good condition.

#1 Sealed bottom brackets are not serviceable or adjustable. When new in the box they are always stiff with a lot of rotational drag. Stiffer than a conventional bottom bracket that has been properly adjusted. Sealed bottom brackets have drag from the grease that they contain, AND the adjustment it fixed so if manufacturing positions them too tight, there isn't any way to adjust the bearing play. There is no way to tell a good one from a bad one when new in the box.

#2 Sealed cartridge bottom brackets are HEAVY. Heavier than a conventional bottom bracket that sells at the same price range. When you jump to the $80 and higher range, sealed bottom brackets get lighter, but are still heavier than a $30 conventional bottom bracket. When I was on a kick to replace my conventional bottom brackets with sealed bottom brackets, the UN51 was about $25, and the UN72 was about $75. The UN72 was easy to identify with it's hollow chrome plated spindle and lighter weight. I also think that there was more attention to assembly, as the more expensive sealed bottom brackets spin lighter than the cheaper UN51, but still not as well an adjusted conventional bottom bracket.