My early-production Tremoverb (1992?) had the switch, too. It's interesting that Mesa has kept that option going for quite some time.
I can relate - I've always been into 6L6s with a Mesa amp for some reason; same with my Two-Rocks. Yet I love an EL34 sound, and my current favorite amps have a British flavor (HXDA, DG30).
One thing that sparked me to try the EL34s is that I love the Mesa Fillmore 50 I have, and it's purely a 6L6 amp. I actually prefer the Fillmore in several ways, and since getting it have used the Lone Star a lot less. So I figured I'd take advantage of the tube choice to make the Lone Star sound a little further away from the tone of the Fillmore.
The Lone Star owner's manual suggests that since most EL34s made today are less reliable than 6L6s, it's worthwhile running the amp on its 'Tweed' power switch setting, which is actually a variac that sets the amp to run at 90 volts internally instead of 117. That way the tubes are, in the manual's description, "coasting." Yeah, I actually read the manuals that come with my gear.
However, the Siemens tubes are NOS, and are probably made a little better. Still, I'll probably run the amp on the Tweed setting most of the time, since it also sounds good.