I agree and this is one of the reasons that prevented me to make a mining company (bonds) on GLBSE. With Pyramining you don't buy assets, you rent them, so you don't lose any value.
Until the account is active, you will get dividends + part of the deposit. Once the deposit is paid back (~10 months or less), hardware previously bought with it is kept at work for other/new members. Because of this, increasing difficulty or bounty halving should not be an issue.
The availability of new technologies does not affect the value of the deposit/investment because it gets paid back anyway in its whole. Should ASICs (or a cheaper technology, compared to the one in use) come out, new hashing power would be bought with that technology and every member benefits from this.
Nothing prevents any member to deposit again if they want a longer term investment.
The referral system allows everyone who invests a bit of their time to earn more than the standard 5-10%.
The only drawback is that an account can't be closed earlier than planned, because the deposit is entirely used to buy infrastructure. Third parties however are already offering to buy for a small fee the outstanding position, offering insurances and so on (google for it).
This method is a win win for both the mining company and their investors, and as far as I can tell I don't see any drawback in case of bitcoin value changes, mining difficulty variation, technologies, and so on.
If you manage to find some referrals it becomes an high-income investment. If you don't, you get 10% (5% if you don't have any sponsor, but it would make little sense ;-) ), which is smaller than piratewhatever operations, but it's plain safe and you know what your bitcoins are used for.
To avoid repeating everything, take a look at the official thread for more details: [https://bitcointalk.org/index.php?topic=80845.0
I would appreciate any comment/thought, expecially in case it could help improoving the service.