IMHO, a sump would be especially beneficial on a small tank, even more so than for a larger tank. Some reasons would include:
1) sump would increase your total water volume. For a 15g display, a sump could increase volume by close to 100%?? More water volume should translate to more stability.
2) sump gives you a place to put stuff, like heater, skimmer (an in-sump skimmer would be much better than HOB), carbon, GFO, etc.
3) water movement between display & sump provides aeration.
Of course, adding a sump does increase complexity to some degree, and also cost. But, IMHO the benefits outweigh the added complexity & cost...
HTH,
Rob