Same here. Been like that since the beta. Can't tell you how many times I've recommended that people update to some version of 7.
I have PVs that are 17 days old.

Interesting. From my small sample:
If there's a WU where there's a PV, an error, and an in progress, it's around 99 out of 100 where the error is from a 6.10.x machine.

I do not know exactly how things work at WCG servers, but isn't it possible to dynamically set the daily task limit per GPU for a given host.
When a host (example my MARS machine) returns crunched WUs back to the WCG servers, GPU/CPU time and elapsed time are known (they are identical in my rigs). Over say 3 days an average value can be computed. From that average value can be calculated the daily task crunching capability of MARS and then the server would allocate a daily limit for MARS.
In this way every host would have its own daily limit tuned to its average capability. When I set the cache size to say two days then the server knows how many tasks to be sent (if available).
If that host gets upgraded then its average will increase and the limit would be adapted accordingly. Some hosts would have very high daily allocations but others very small. In average over the whole grid there should be no change and there would be less hosts waiting for WUs. To avoid continous adaptations we could set a threshold (example +10%) that would trigger the daily allocation adaptation.
Does this make sense or my reasoning is flawed somewhere.

The daily quota shouldn't be a problem, since AFAIK it's already so high that unless you're generating any errors you shouldn't hit this limit.

As for giving you upto 7 days of cache on the other hand is not such a good idea. The problem is a fast GPU can do 1000+ tasks/day, meaning a full 7-day cache would be 7000+ tasks. If you have 1000 such computers, you've got 7 million tasks in the database, and with so many to look through all database-traffic will go slower.

By limiting cache to example 500 GPU-tasks max at once, a computer can still crunch 1000+ per day, and 1000 such computers can still crunch 7 million in a week. But, at any given time there'll only be 0.5 million tasks in the datase, if no grace-period and everything is immediately validated.

With a 1-day grace-period and some random waiting for validation it will probably be somewhere between 1.5 - 2 million tasks in database at once. While large it's much less chance to run into database-performance-problems.

What and/or who was that in response to?
BOINC has no problem with a miriad of other projects being able to be granted credits. If in fact this is an issue, HCC1 should find a way to increase the 'run time' of their WU's. If HCC1 cannot for some reason differentiate CPU from GPU..... Their bad. Learn to!!!!!!
If it's all about the science, let's let it be that way. GPU's are faster, get rid of CPU WU's. There's HFCC for them. What? The HCC CPU'ers don't care about children????

CMON, this argument is getting stale;-(