While I agree with the concept, I'd rather just have the money now on the medical expenses. Once the three years has passed you can toss the receipts.
A bird in the hand is worth two in the bush.
The extra money now lets us do more... but then we are already investing a significant amount for retirement/long-term as well as saving/spending more on the non-long term. Not including the HSA, since it's still in the cash accumulation phase, we are putting in over $66k in 2016... by far our highest amount ever in one year. More than double what we put in during our first year using [the other budgeting software], and 5 times what we put in the year before starting [the other budgeting software].
[the other budgeting software] has helped control our spending which has allowed us to save/invest more in general.
ETA... our HSA investment options are pretty limited. The only investments of interest are Investor Shares of Vanguard 500 and Vanguard Total International.