It's good to secure one such long-term visa because Thai policy keeps changing in Thailand.
At least for the peace of mind in the 5 years, for those who love Thailand so much or having long-term education or planning to set up a second home there or having a new relationship.
A student visa can only allow for a 90 days stay for a single entry.
Extendable or not it's up to the immigration office.
What if the immigration refuses to extend for you and you're halfway done in the courses that you've paid will also be an issue.
If there's a need to extend every 90 days, it's a waste of money and time to go for a visa extension if you can get DTV for free of charge and hassle free(Singapore's citizens).
I'm a Singaporean. The Royal Thai Embassy in Singapore has not approved by the Thai authority to issue DTV for Singapore passport holders, that's why I've to apply it abroad at the nearest embassy/consular, which Malaysia.