I'm using a second SIM only because I can; like you, the T-Mobile coverage in my area is on par with both AT&T and Verizon (perhaps just a touch less, but all 3 have various dead spots here).

If a couple of other phones in my family have a need for secondary coverage, I might go back to Xfinity's $12/gig for the month; that's actually not bad spread out over 2-4 lines that won't typically use anywhere near the 1Gb, but do have the option if there's ever a need.

What does the 3 UK SIM get you in the US? Isn't it only valid for 30 days?