slidelock reload: the empty magazine hits the deck and never comes back.
speed reload: when I want a fully loaded gun but my ESP is broken, I’ll drop the partially loaded magazine on the deck and insert a fresh one. I’ll pick up the downed mag if time permits but a mag that’s hit the ground is suspect and I wouldn’t want to rely on it without a chance to examine it first.
tac load: if my ESP is working, I tend to do the “at the gun” tac load simply because I’ve been practicing it forever. The fact that the other method is faster is meaningless because my ESP only gives me the green light to tac load when I feel as safe as I do right now at home typing on the internet. 
Equipment and circumstance both influence these things. For about a year when I was carrying a SIG P220 and 17 total rounds (9 in gun, 8 in spare mag) the idea of retaining a partially loaded magazine took on greater importance. This was especially true since the gun, after firing just a few rounds, was at half capacity and prudence dictated filling it back up. Cf. my current everyday gun, a S&W M&P9: if I drop my fully loaded mag on the deck before the fight even begins and reload from the belt, I still have more ammo (18rd) than I carried in toto with the P220.
In 15 years of studying this stuff, I’ve still never seen documentation of a single instance in which a civilian or domestic LEO affected the outcome of a fight because he retained a partially spent magazine. The fact, most folks carry far more ammo than they’re likely to need in a single incident and spare ammo, mags, etc. are waiting for them back at the house, station, patrol car, etc.
I’m also very skeptical of the whole “lull” thing, as you may have guessed from my “ESP” comments, above. We ran a test once at an IDPA match with about 50 shooters. Skill level ranged from Novice to Master. (You may scoff at IDPA being just a game, the fact is that a Master-ranked IDPA shooter probably practices tac loads a lot more than most cops and soldiers) Anyway, the scenario basically called for the shooter to perform a tac load behind cover at what he believed was the end of the course of fire. However, once the partial magazine was ejected from his pistol a surprise target popped up … in other words, he was interrupted during his supposed lull. Every single shooter finished completing the entire tac reload sequence, including stowing his partially loaded magazine, before firing on the target. I’ve had people argue that “in a real fight” someone would be switched on enough to react properly. Sorry, I’m a believer in “fight as you train.” When we practice a complicated multi-step action like tac loading, we don’t do it in little steps. Our brain says “tac load!” and the body goes through all the motions. If you’re not specifically practicing to change direction halfway through the technique, I don’t believe you’re likely to do so under stress. So you’ll be performing that administrative reload while someone is firing rounds at you.
I’ve also seen this in Sims/FOF training. When rounds may be coming your way at any moment without warning, there are no lulls. You know how much “lull” you just sat through, but you have no way of knowing whether that lull is going to last another three minutes or three seconds. But if you’ve already fired a bunch of rounds (“bunch” being relative to your mag size) you might want to have your gun at max capacity right frakking now. This is where the speed load comes in.
For CCW, my take on the tac load has always been, “carry another spare mag and don’t worry about it.” If you’re really that concerned about leaving half a mag’s worth of ammo behind, just carry another full mag with you.
For a LEO this might be more complicated, as duty belt real estate is at a premium today. Still, most LEOs carry at least two spare high-capacity magazines. Beyond Revenge of the Zombies it’s hard to imagine a realistic scenario in which both (a) you’d need more ammo than that and (b) you’ve got a chance of winning. Half a mag’s worth of ammo isn’t going to turn the tide if the Mongol hordes come over the wall.
In a .mil context, I absolutely appreciate the logic behind keeping partially depleted mags and perhaps even empty mags when you can do so. But there are some key factors in that context. First, you really may be far away (both in miles and days) from any chance of resupply. Second, you are probably much more likely to engage in multiple, possibly unrelated firefights between resupply. Third, you are much more likely to engage substantially numerically superior forces. Fourth, you are almost certainly operating in a team environment providing you with time and cover to perform a tac reload.
Equipment and circumstances … believe it or not, some of the TTPs of the world’s most elite CT teams may not be a 100% solution for a lone out-of-shape middle-aged CCW’er getting mugged by a couple of gang bangers in a bad neighborhood. Though I admit there have been times I’ve been in Baltimore and wished for CAS. 