Only you can answer the "is it worth it?" question.

I loosely think of miles as 2 to 2.5cpm when I'm considering redeeming for a simple coach award where I'd otherwise have to pay cash. That roughly guides my "buy vs. redeem" desires, although other factors certainly play into it. (My most common UA use after long-haul premium cabin: short U.S.-Canada trips that are typically high-cost and short enough where I care less about EQM's and upgrades.)

But I wouldn't buy them for that without a planned purpose.

Someone out there who regularly buys int'l premium cabin for personal trips might place a higher value on miles and buy the maximum allowable per year at that rate.

Someone else out there in a market where they nearly always have access to cheap airfares to their desired destinations may never go over 1cpm. This was me when I lived in Chicago. I could always find cheap tickets for trips I had to take. Therefore, I just hoarded miles for int'l F tix that I would otherwise never buy. So I probably wouldn't have paid over a penny a mile without an immediate purpose.