I play a very slow and methodical strategic game myself, using specific hit and run strikes when I attack enemy planets, pulling my forces back for repairs using an array of MRS units after each strike. When I'm not doing that, I spend a lot of time scouting and being very picky about what planets I take. As such, I'm usually fairly awash in resources, and more often than not I'm at the cap. The only time I'm not is if I've unlocked new ships and I'm building them, or if I screw up my planning and lose my forces faster than I can retreat them. So I find myself using Merc Docs every now and then to increase my defensive ranks, especially in the later game when I'm deep raiding and can't easily and/or quickly move my ships around the battlefield.
They do suck the resources down fairly quickly though, so it's important to know when to use them in case you suddenly need major main fleet replacement. They're not economical at all, but they do give you a little extra 'oomph' on the battlefield. Just don't rely on them too much.
As an aside, my style tends to arise from a background of strategic games where unit survivability is extremely important and reinforcements are limited or non-existent. For example, Ground Control 1/2, Advance Wars, Super Robot Taisen, stuff like that.