You should always join whatever program is needed to get points.
I've joined everything. When hotels or airlines merge, I've benefitted the merges. I didn't have enough Sheraton or Westin. But when they merged, I suddenly had enough for a free night. Delta and NW together gave me a free ticket. USairways and America West, also produced me a free ticket. AA and TWA got me close enough that a car rental put me over the top for a free ticket.
Always join. If UA and US merge, who knows, you might have enough pts.
The one I benefitted from the most was Air Canada and Canadian Airlines. That one got me a free ticket to Asia.