The telomere issue could be fixed by temporary re-activation of telomerase. Regardless, while telomere shortening has been implicated in a number of diseases of old age, no causal connection has been formed between telomere length and aging. Indeed, some creatures (mice come to mind) have absurdly long telomeres and do not reach the limit of their telomeres within their normal life-span. So for now, even with constitutively active telomerase, we have to assume we would still age and die. Either way, it is unlikely to be a great impediment to cloning.
All right then - I was trying to simplify things, but the question remains: How can slowing evolution to a crawl be good for a species' survival?
I see no particular benefit to a slow pace of evolution, but neither do I see a benefit to a fast one. As K selectors we have we have already evolved to evolve more slowly (and generally the slower a population reproduces and evolves as approaching resource limits the less likely it is to suffer dieback). Evolution is not a linear path in a positive direction and a high speed of evolution is not necessarily good, especially after a certain population/biodiversity level. Further it would always possible to amp up the evolution rate of clones by inducing mutagen stresses.
Again, not saying that it is a good idea. I am just saying that I am not sure this particular argument against it is that compelling.