Unless your not going to a much lower level zone, you probably wouldn't steamroll everything anyway, I'd say you keep about 2/3 of what makes you powerful when down-scaled by 15+ levels, you've still got the same weapon skills, you keep your utility and elite skills, so if you do go to a area where people of that level haven't unlocked all of them yet, your still more powerful, and you keep your gear, and all the stats it provides, which is probably half of the most import stat for you build. You really only loss your natural stats and a bit of health when down-scaled, you keep most of the powerful stuff.
As it is, a level 15 zone is easier for a level 15 player than a level 70 zone for a level 70 player, so by the time you get up to the higher levels, you'll also be a better player (in theory). And as was mentioned, if you enter a lower level zone, you don't also get scaled down to quite that level, if a level 70 player entered a level 15 zone, they might only get scaled down to level 17, so they are still more powerful, plus all the boosts I mentioned above.
As an example, as I said in my last post, I'm level 80 now, but I was passing through a started zone, and while traveling, occasionally would just shoot enemies in my path because they have a slow effect (cripple, chill, immobilize, etc) and it is faster than running round them, but in one case, I was traveling through the level 6 part oft he zone, and I was able to 2-3 hit every non veteran enemy, I don't know about you, but I'd still call that steam rolling them. As it was, my attacks were fairly fast firing, so I didn't even have to stop and shoot, and since I'd drawn my weapon, if I switched, I get swiftness as an added bonus, so if anything, I went even faster.