Cons I can think of:
- As the gravity of the run-off from the mash decreases, the pH increases (towards the pH of the sparge water) and with it the chances of leaching astringent-tasting polyphenol compounds (tannins) from the grain, especially at high temperatures (>176°F). If you're after every last bit of sugar, you could possibly do more harm than good by over-sparging.
- Assuming your mash bed has run dry, you may have to recirculate any water you add to get sufficiently clear wort. If it hadn't run dry, I assume you'd just use that to top up.
- It takes longer than just adding water, but not significantly more.
Obviously the last two are fairly minor, and even the first will probably be of limited consequence.
I should note that this situation is always a tradeoff between volume and gravity. If you're below the desired pre-boil volume and the gravity is lower than expected, topping up will exacerbate the problem. If, however, the wort is stronger than expected, you could top up. Personal preference will dictate what's more important, the volume or the gravity.
It's easy to estimate how much water you need for a specific pre-boil volume based on how much grain you'll use. Every pound of grain will predictably retain roughly 1/8 gallon of water, so for any particular grain bill:
[Total water] = [Pre-boil volume] + ([pounds of grain] * [0.125 gal./lb.]) + C
where C is your system loss (unrecoverable liquor left behind in the mash tun/tubing, etc.) You'll have to figure this out yourself, but it'll probably be between 1/4 and 1 gallon(s). For example, if you want 6 gallons, pre-boil, are using 15 lbs. of grain in the mash, and know your system retains ~.5 gallons:
6 gal. + (15 lbs. * 0.125 gal./lb.) + 0.5 gal. = ~8.4 gallons.
I'd add half a gallon, just to be safe (more if you don't know your system loss yet). This could help avoid coming up short, assuming you don't do this already.