It's like Chinese whispers, each time a cell divides, the content of the genomic information changes. DNA replication has error checking, but it's not 100% able to copy the genome entirely as it was.
Telomeres are your cell's mechanism to track how many times a cell has divided, and in essence how many DNA errors a cell will contain.
Limiting cellular division by shortened Telomeres protects you from cancer. If you artificially introduce Telomerase genes into a cell line, sure, you will have more new cells generated, but their genone will be 'old', containing more mutations.
Really, this sort of treatment can only work in tandem with treatments which resolve genetic mutations and errors. Until we have the ability to fix our aging, mutating genomes, then simply lengthening Telomeres artificially will lead to diseases associated with genetic mutations, such as Cancer.